Here are a few of the books on my bookshelf that are not available as ebooks for purchase and would probably be very hard to find pirated copies of:

    The Psychology of Computer Programming
    The Paralation Model
    Handbook of Human-Computer Interaction
    Object-Oriented Programming in Common Lisp
    Computer-Aided Financial Analysis (Miller)
    Probability, Statistics, and Queueing Theory (Allen)
    The Brain Makers

Note that I sold off most of my library and regularly sell books once I am done reading them, so this is just a small sample of the material that you will not find online, that I have personally read. Also all the older/rare books I have borrowed from/read at my local public library (public libraries have a surprising amount of computer books), and the university libraries I have been associated with earlier.

Whoever discounts computer science books today because "you can find it all online" does so out of a very deep ignorance of computer science. Between Google Books and the Internet Archive's scanning project digital copies do exist, but are inaccessible.