I'm quite generous with allowing use. Since my books are mostly educational, if people credit me, I allow them to use information as long as they note the name of the book and that I am the author. I would be upset to find information had been lifted from my books without permission or acknowledgment, though. I self-publish so have complete control of the process. Also design my own covers, except for one book. I gladly give credit to the artist who designed that cover, Brian Burak. He was an art student at the time and charged me $20. I hope he gets better rates today :).