Unfortunately, authors now have to compete with people who write online articles (which is basically free for people to read). Not many people write books for chess variants anymore, and they're all out of print. One example is below, but since it's now "rare" it's expensive. There's five available of this, and the cheapest is $191.
A pdf of the second edition of 'The Encyclopedia of Chess Variants' is freely available at http://www.jsbeasley.co.uk (click on the 'Chess Variants' section).
Another book is 'A World of Chess' http://aworldofchess.com (mainly about variants, especially historical ones) which is about $50 on Amazon, but the digital version at Google Play Books https://play.google.com/books is much cheaper.
Hello my dear friends.
I have started getting interested in Chess variants.
While these games are somewhat similar to Chess, they also have profound differences, in stuff like
the shape of the board, the identity of the pieces, the patterns of movement and capture of pieces,
some variants( like Crazyhouse) even allow you to drop back captured pieces.
And thus these games are immensely interesting and merit specialized printed books about them,
that deal with stuff like annotated games, opening strategies, puzzles etc.
Examples of Chess variants on which I want to see specialized printed books that currently don't
exist: Crazyhouse, Glinski's Hexagonal Chess, Rhombic Chess, Masonic Chess,Capablanca Chess,
Grand Chess.
I have emailed some chess book publishers like Gambit and Thinkers Publishing asking them to publish printed books on chess variants.
Perhaps you can do the same and maybe find some other avenues to promote the publication of
printed books on chess variants.