As Peter said, large books aren't really a problem. One of the firmware versions had problems if you tried changing the fonts to many times. This seems to be fixed in 2.3.1.
I thought the dictionary worked with all formats. But, I just tried a TXT and PDF and it didn't. I am surprised at the TXT not working, but the PDF might be its formatting. Some are really just images and some are text. The PDF I have on my device is a train timetable, so it might be the image type.