BE-300 can be used for reading books, which is my personal reason for buying this little toy.
Even with installed memory you can fit at least a dozen books in zipped format. On top of that, CF memory cards are very cheap, you can pick up a 512mb card for only $25 (with rebate), which means you can literally store over 1,500 books on this tiny device. With a 1gb card this amount doubles to 3,000!
Another reason for preferring BE-300 is that many programs that work on PC can be used on BE-300, too; for example, Internet Explorer - a very shrunk version, doom II - to be fair, it may be very hard to play, more on that later, tombo - an outliner notes program, and nethack - haven't tried yet if it'll work.
I was worried about the screen size, but it turned out that BE-300 screen is perfectly reasonable for reading large books. I've read 4 or 5 of them already, at a very quick pace (I only have it for couple of weeks). Keep in mind that most of the reading was done on subway, at only about 3 hours a day, so I think it was a rather good reading speed. Also keep in mind that I'm not an exceptionally quick reader.
The best known ebook site on the 'net is http://www.gutenberg.org/. You may also be able to find more ebooks at alt.binaries.ebooks newsgroup sometimes, but you need a newsbook reader program and your ISP must carry newsgroups (most do). This site has many dozens of good books about Yoga - fully available for download: dlshq.org