I had to do the terminal # omit trick in sudo nano to get past iOS update error dfu mode of 4.2.1 install. But greenpois0n, cydia, fake operator, ifile, mxtube, sbsetting, mywi, many others all seem to work great.
All of these work on my iPad with greenp0ison. I can open books in iBooks but I don't have any DRM books in it that I know of (just the free ones so far).
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148 Safari/6533.18.5)