Apparently, the book For a Lost Soldier has gone out of print and become rare. Looks like you can still find it if you search through used book sellers but the cheapest I saw it in a quick search is $188. But if you really want to read it, you could always get it at a public library. If they don't have it they can usually get it through an inter-library loan. It's a good read and paints a far more complex picture than the rosy one portrayed in the movie.

I'd also recommend getting "Loving Sander" if you are able to. It's also out of print but not as rare as For a Lost Soldier. You can still find some copies for under $30 (used). It's my favorite BL book. I believe it paints the most realistic BL/YF relationship of any book I've read; I'd be surprised if the book is truly as fictional as it claims.