I found this thread on the WF about how to get Public domain works very easily, and how these books or writings can benefit today's INternet Marketers.
Here's the thread:
How to claim your FREE (unexploited) Public Domain work!
I Often use Public Domain works when making new info products. The thing is, you have to make sure none of the works you use have a copywrite on them. So if you check this out, keep copywrite notices in the back of your head.
You will also have to change the content and titles too. At least that's what I've found that works the best.
Have fun with it ....