I have seen a lot of commercial Ebooks for the Blackberry lately. However, what I have been doing in this regard is free and works perfectly on my 8700g:
Go to The Gutenberg Project
on your desktop/laptop computer
Find what you want among tons of free books. Download the zipped html version of the book. Email the ebook to yourself and open the zipped attachment on you Blackberry. Voilą! You've got a nicely formatted Ebook displayed in your browser. You can leaf through it while reading by hitting "Space" which takes you down one screen of text at a time. It gives you a comfortable continuous reading experience.
I have read a lot of books this way when I travel and elsewhere, and have had no problems with it.
If this is known to you guys I am sorry for ranting about it. If not: Enjoy!