manybooks.net is great !!
I have it listed on my
Freeware Post at PDACorps.com, along with many other great sources of free eBooks.
Project Gutenberg is probably the largest source of free eBooks in .TXT format
You can easily create your own ebooks by saving them as a .TXT (plain text), .RTF (rich text format) or .HTML (webpage format - hypertext markup lang).
And as mentioned above, you can just open a .PDF file in Adobe Acrobat, and then save it as a .TXT file
And remember to convert your .TXT to compressed .ZIP files before loading to your PPC. uBook can read the ebook from within the .zip file. It will save you lots of space.