I went with 3 free ones to start. Some CC from bittorrent (not copyleft, but CC... so legal torrent) that started on kickstarter, and one free from lulu is a decent start. I was hoping you had done some reading and had recommendations.
Artificial intelligence for humans vols 1-3 was a little weak, but it was CC and free. So, what do you want right?
I don't remember which one was free on lulu. Might have been a field guide to genetic algorithms