iTunes was required to sell hardware. The iPod, iPhone and iPad all required iTunes in order to function, so iTunes on Windows was not something they could avoid.
There is no such motivation with iBooks.
I have no idea why they decided to support Safari on Windows, however some of the early analysts back in 2007 speculated it was to give PC users a taste of the Apple world before the release of the iPhone, so they would not balk at having to download Apple software - iTunes - in order to initialize and use their iPhones.
They were targeting "switchers" with a teaser about the previously unknown Apple world.
Others stated it was in support of Apple's foray into the web software market - which they basically abandoned, leaving only iWork.com behind.