This is likely to be more personal conjecture with a basis on the industry knowledge I've picked up from years of working in book stores, reading to far beyond the point of addiction, editing, manuscript reviewing, studying etc. etc. than anything else. There will be no real research done as I'm already feeling like I've been dragged backward through mud and doing research at this time will just make me feel like someone flipped me over and changed directions. So please excuse the informality.
Well, if ever I'm going to get away with it its here on a blog.
Over the years I've noticed something odd that sets apart the male and female readership groups. And before I mention it, neither one nor the other is the best pattern for reading as the best pattern is entirely personal and up to you. I'm not going to push you to read anything, except maybe a dictionary and a few science books and Chaucer for a laugh.
So, the odd thing I've noticed, which may impact me directly someday, is that male readers and female readers do not choose books in the same or mirror pattern according to their genders. I'll probably have to explain this...
Male readers often choose books that have a male protagonist at the fore and rarely stray from this. They do stray a lot more when it comes to movies as there is much visual appeal involved but even so the female lead they are watching generally has to be a kick-arse lead or a lady with a boomstick. (See Evil Dead II for the meaning of boomstick if you haven't figured it out. And even if you know, go watch it again. With a beer. And chips... Okay enough dreaming on my part.) Even those men who are homosexual still prefer to read about men than women as their lead characters, meaning that there really isn't much of an overlap in audience unless the book is famous, neutral as there's a male and female lead given equal footing or the subject is appropriately appealing in some hard core manner or another. Almost never will you see a man wandering the romance section to buy for himself, let alone be seen wandering the romance section to buy for someone else. Online order of such things only. Men will, however, freely buy very pink and glittery books for their children or young female relatives as such books are obviously not for them.
Female readers are a different matter entirely. They're reading habits rarely mirror the above patterns at all. Female readers almost never stick to female protagonists only even if they do manage to amass a collection highly female protagonist oriented. Instead, female readers often pick up books with male leads, read texts with hard core subjects and dive into neutral stories with ease while at the same time can be seen wandering through romance sections, poetry sections, any manner of hobbies including knitting (my father knits by the way and I'm always shocked when men get frightened off by appearing feminine knitting - my father being rather a muscled and tall man holding knitting needles and knitting jumpers didn't look at all feminine) and sewing, fantasy, erotica, home building, architecture, gardening and children's sections. Female readers read pretty much anything with their own personal preference for protagonists and genres taking the fore but not nearly excluding other types of books as male readers do in their collections.
Of course, this doesn't apply to everyone as that personal preference always comes into play when someone's buying books.
I'm not sure how this difference comes about, whether it is through habit, getting used to reading male leads in classics when in school, curiosity as to what the other bunch think or a willingness to accept both the masculine and feminine within more than men do but females definitely differ in their reading practices. To me, it is interesting enough that I continue to watch people buy books (a little voyeuristic, I know) with great curiosity. And it always strikes me than men aren't nearly as relaxed in how other's perceive their book selection as women are. And that's coming from someone who does try to hide the rather girly covered book in amongst the less girly purchases so only the teller knows my shameful secret.
How this impacts on writers and publishers is easy enough to see. Marketing is difficult for one group and easy for the other due to the difference in book selection trends. Writing a female protagonist means you're left wondering if any significant number of males are going to read your work and if they aren't then do you have free reign on idealising men in your book. What wouldn't work at all in books selling to men might be perfectly fine in books selling to women. If a woman writes a male protagonist they can expect a greater readership than if they write a female protagonist in a similar role. But should you change the characters gender to gain that readership share? Well, when writing you often find only one character truly fits the mould so if that character happens to be female then you're stuck unless you want to compromise. While female protagonists are multiplying by the dozen the readership still limits their spread to niche.
Even if there are first and third person female protagonists as the lead characters in the vast majority of books sold they might still have a strangely skewed readership when gender is taken into account. I'd hope not, I'd hope that the male readership would cut their losses and end up picking up books with any sort of protagonist as long as it fit with their genre preferences but I'm still a little doubtful as to what would happen is such a case. Well, I also doubt such a case will ever come about so we'll likely never know.
There is one thing I'd like to say though, with regards to the expanding number of books with female leads. The male readers out there are missing out on some very good reads by avoiding them. Granted, in amongst the number are those with idealised male characters that will likely make you want to either cry or shoot something, those revolving far too much about romance and those with overly wishy-washy female characters that just need a good kick up the butt and slap of realism but there are still many good books for male readers with female leads about.
Here are some non-classic (mostly) suggestions for books I've rarely see men read:
Lilith Saintcrow's Dante Series
Completely hard core and not a wishy-washy female character in sight. More straight horror than anything else but many bookstore will shelve it under paranormal romance, which always has be cursing in frustration at the poor marketing (a sales rep or distributor recommended such shelving).
Kristin Cashore's Graceling Realm series
Some rather extreme action involved plus personal discovery that isn't solely to do with romance. Also, sword fights!
Markus Zusak's The Book Thief
Classic in the making and already sharing fairly even readership as one of the characters is Death but still, I'll suggest it anyway to get you started.
Ilona Andrews' Kate Daniels series
Some romance involved but it isn't overly idealised and the action is dominant and bloody. Also, the number of male characters is greater than females.
Kelley Armstrong's Women of the Otherworld series (there's also a book on the men at one point)
If you going to really get into female protagonists and try to discover something vaguely realistic on the romance front then read these. They're also full of blood and gore, pack politics and monsters so they're fun for anyone. I also found these books to include fairly believable male characters, once you got past the werewolf front. And another also: I found that Clay is one of the most well-written male werewolf characters out there. He's more wolf than human and a fair deal of wolf behaviour patterns have been included. Does include steamy sex scenes but at least the women aren't lying there going ooh, aah... (I'll shut up now).
This series is often picked up by both male and female readers alike but I'd like to mention it for the odd reactions it gets from both. As fantasy readers male readers will pick it up because it is really fantasy and quite uniquely so. As female protagonist readers females often pick this up for being one of the few fantasy series out there with female leads.There is an overlap for female fantasy readers and male female protagonist readers but not as much as I'd like and I could never recommend this series enough. Have fun and read it whoever you are.