iBooks is a bit easier to navigate with the addition of 3D Touch, particularly if you are one to take notes or add bookmarks.
You can press as hard as you can while reading a book but you won't get anything other than the same Copy/Define/Highlight/Note pop-up menu as before. I was hoping, for example, that I could 3D Touch my way to the definition of a word but that's not the case. Instead, 3D Touch comes into play on the Contents, Bookmarks and Notes tabs as well as the search results page.
On the Contents page, you can press to peek at the first page of a chapter, which can be a page of text or an image. Unfortunately, I found that the peek feature can sometimes show you a blank page if the book's chapters are separated by blank pages.
On the Bookmarks page, you can peek at each of your bookmarked pages, which is useful if you have lots of pages bookmarked for a book you are reading for a class and need help keeping them straight.
Even more useful perhaps is the ability on the Notes page to peek at the pages where you added a note.
3D Touch can also aid your search efforts. When searching for a term, you can press to peek on any of the pages listed on the search results page, which saves you from having to tap to open one of the results listed and then tapping the magnifying glass (or the font or bookmark buttons closely flanking it) to return to the results page.
In all four instances above, you can go from peek to pop. That is, while peeking at a page, you can press harder to pop out of the peek view and pop open that page.
For more, see a list of all of the changes included in the iOS 9.2 update.