Lifehacker notes thousands of scientific titles are available for free PDF download from Springer.
There’s a bunch of titles on evolution.
And they even have Whit Au’s “The Sonar of Dolphins” available. I don’t mind paying $130 for my 1993 print copy, but a free PDF is a fine thing, too.
Update: It looks like the free downloads of books are over. If it was a mistake at Springer, then we probably won’t see that again.