There are two paper books, neither of which I've read, but which have been well recommended here. One is by Rob Sylvan, the other by Ellen Boughn. Both seem to be available for Kindle.
Other publications seem to be available, I have no idea how useful any of them would be to you.
There is a purchasable pdf download here which would probably be useful:
http://rasmusrasmussen.com/microstock-photographers-guide

And, to be shameless, there is my book available at http://www.backyardsilver.com/stock_photography_ebook/ as well as Amazon and the Apple store

Steve

OK, this is my first post and I don't make my living shooting stock, so take this with a grain of salt. But, I do have a lot of experience working at agencies.

Study the sites on the right. Many have useful blog articles that discuss trends, highlight what's selling. Some sites show # of downloads, views, popularity. Study it. The agencies want to make money, and it's in their interest to educate you so you create photos that make them money. They make money, you make money.

This forum has some very good perspectives, but in my experience it seems like it's used mainly to discuss the pros and cons of individual agencies (royalty rates, reviewer standards, sales overviews). There seems to be less focus on guidance about how to carve a niche and create good content. That's not a bad thing (obviously users here aren't going to divulge all their secrets), but keep it in mind.

This is mostly aimed at people shooting editorial: the BJP writer David Kilpatrick who is also a stalwart of the Alamy forum has a good article on his website - full of solid common sense advice and reminders:

Thirty Key Points About Stock Photography

First rule of stock photography - don't spend a lot of money on books, equipment etc.