If your eBooks are already in PDF format, just email them to yourself and you can view them as an attachment (Blackberry supports attachment viewing of PDF, Word, and Excel files)

One could also use one of the Dynoplex software such as eWord to view Word documents on a BlackBerry.

The thumbwheel on the Blackberry and configurable font sizes (Try "Millibank Tall", 10 point, my favourite font) will make eBook reading a pleasure. The problem is you're limited to PDF, DOC and TXT eBooks at this time, to the best of my knowledge.