The WSJ's Lee Gomes recently wrote an article about reading books on a Blackberry. It's something I originally tried on a 7100t but met with disappointing results. My 7100t died last year though and I wound up switching to the larger 8700g (shown on the left).
Ever since then I've found myself using the Blackberry for more reading than ever before. It's amazing how much the reading experience improves when you go from the 7100t screen to the slightly larger 8700g. Besides the larger screen, the major reason I find myself reading on a Blackberry is the reason Gomes cites in his article: You have the thing with you, so you might as well make the most of it.
I can't tell you how many times I've stood in line in a store, pulled out the Blackberry and started reading. One of my favorite selections: Book summaries from getAbstract, which I might add are available in a simplified PDF format that are ideal on a Blackberry. In fact, I recently read two summaries while waiting for an oil change; now that's productivity!
Other content sources I've dabbled with include DailyLit and Mobipocket, which also offers a free Blackberry reader.