The Phone Dev Team's PwnageTool, which allows for the Jailbreak of iPhone, iPod touch, iPad, and Apple TV, has been updated to version 4.2 and now is untethered and fixes issues with iBooks.
With [Chronic Dev Team's] permission, we’ve incorporated their 4.2.1 “feedface” untether into today’s PwnageTool 4.2. This means iPhone unlockers can safely restore to a custom 4.2.1 pre-jailbroken IPSW and retain their current baseband and unlock.
PwnageTool also includes two very recent improvements to the 4.2.1 JB: iBooks was just fixed by @comex and @pushfix last night so that it works as intended on DRMed books, and the wifi problem on AppleTV 2G was fixed by @nitotv, @DHowett, and @saurik. Both of these fixes will also be available in upcoming Cydia package updates, so if you’re already jailbroken you can wait for those updates rather than restore and jailbreak again.
If PwnageTool is your preferred Jailbreak, go get it via the link below (they have mirrors posted as well) and tell us how it works for you!