You can easily delete books you have added to iBooks unless you have added them from the iBooks Store. In that case the best you can do is to hide these books using a hard-to-find function in the iBooks app.
While e-books in iBooks are not supposed to be a substitute for audiobooks, it is possible to get iBooks to speak text to you. If you select enough text, you can make this an occasional alternative to reading with your eyes.
You can highlight text in iBooks and add notes for each highlight. These can be used as study guides or simply reminders. Highlights and notes in iBooks textbooks will be added to the flash card study function as cards. You can also export the highlighted text using copy and paste. They also sync across your devices using iCloud.
Take a look at iBooks reading options on the iPad. WIth iBooks 3 you can change the font, font size, background style and even switch to a continuous scrolling mode. Each style has its merits depending on what you like, the size of your iPad and whether you are holding it horizontally or vertically. But textbooks and PDFs will not give you as many options.
The new version of iBooks for iPad allows you to read advanced textbooks. These can include interactive images, 3D objects and even review quizzes. You can add your own notes and then review them as Flash cards.
Take a quick look at iBooks Author, the new tool from Apple that lets you create textbooks for the iPad. You can design and layout pages like in iWork Pages. You can also add special elements like photo galleries, quizzes, 3D objects and interactive images.