What do you think a "gender neutral" book would be? I don't think an all-women book or an all-men book is gender neutral. Obviously there are lots of other ways books can be "coded" for gender.
One that doesn't specifically take the stance of "This is a book that examines Issue X from a MAN'S perspective." or "from a WOMAN'S perspective." Most books, I think, are really gender-neutral stories, but we choose to label them as "for men" or "for women" based on the sex of the characters, which is making the totally silly assumptions that 1) women won't enjoy stories with male characters, and vice-versa, and 2) that just because a character is of a particular gender, that book MUST be about the experience of that gender. There are many books, in fact, that are simply stories of HUMAN experience without bringing into it how that human's gender changes his or her struggle.
Let me give you some examples from the other side of the fence, so you can see what I mean about gender neutrality by an example of contrast. Two of my favorite collections of short stories are The Dog of the Marriage by Amy Hempel and Male of the Species by Alex Mindt.
Although most of the narrators in Hempel's stories are women, most of these stories are about emotional struggles that both men and women face -- specifically, the dissolution of relationships (it's a common theme throughout the collection.) I consider The Dog of the Marriage to be gender-neutral, since the focus is on the emotion of loss in the context of relationships, not on how women uniquely perceive loss of relationships.
By contrast, Mindt's collection is about what it's like to be a man; what unique struggles are experienced by fathers, sons, brothers; what it's like for a man's perspective on life to change as he ages; and other stories that are specifically written to view the world through a man's eyes, in ways that directly speak to society's views of maleness. I do not consider Male of the Species to be gender-neutral. I still love it.
I haven't read The Heart is a Lonely Hunter,
Then why did you automatically assume that it's got no female characters, or even that it has an all-male cast? That was a funny sort of knee-jerk reaction you had.
but I have read Flowers for Algernon. A book with no (or a few incidental) female characters is not "gender neutral."
I really don't give a shit how many characters of any gender are in a book or a story. What makes it gender-neutral or gender-biased is the CONTENT AND THEME OF THE STORY, not its cast. Flowers for Algernon is about a person's loss of self as that person becomes more acceptable to society, and about how society negatively judges those who are different and tries to change them. That is a theme that spans gender. And if you (general you) have a problem connecting with all-encompassing human themes because your plumbing doesn't match the character's, then I question whether the empathy center of your brain functions properly.
That doesn't mean the story and characters might not appeal to both genders, but having no protagonists you can identify with unless you pretend you're the same gender/race/etc. is a big part of what makes a lot of books and movies unappealing to overlooked groups.
I truly feel sorry -- truly; this is not sarcasm -- for people who are so self-centered that they cannot identify with other human beings, no matter what their gender or skin color or socioeconomic condition. We are human beings first and foremost. I have no problem immersing myself in a story that has all-male characters, all-black characters, all-wealthy characters, all-French-speaking characters, or whatever. I believe there is something sadly un-nurtured in the minds of people who must have a particular identifying character who is just like them, and they cannot connect with and understand a story about people who are different from them.
There is something really, really wrong with a society that raises its children to believe that they can only empathize with those who are so similar to themselves as to be indistinguishable.