The best feature in the new OS would to me be hands down the folders. I am now able to better organize my apps, and know where all of my favorite apps are. An added bonus to the folders is that I am able to fit many more apps on my phone, as I have had my phone full from a few weeks after I got it until last night. I now actually have empty spaces on my home screen.
Another great feature is the new inbox setup. I am now able to check all inboxes at once, as well as independently, saving a lot of time.
There are many other minor tweaks, such as the option of choosing different search providers in Safari, and minor graphical redesign among them.
iBooks does not come preloaded with iOS 4, but it is available for download from the App Store for those who have upgraded to iOS 4. The app is a great eBook reader, with plenty of free books available among the paid books.
The biggest surprise I encountered from iOS 4, is that it does not slow down my device. I expected my device to perform slower, as it is a few generations back, but it performs nicely on the new iOS.
I would recommend that all iPhone users (3G and up), at least try the new iOS, I don't believe you will be disappointed.