There are many Libraries on the Purdue campus, but the Books and Media on the Libraries website searches the collections of all, including eBooks. Boosk and Media search results will tell you where books are located, and if they're available to check out.
If the Libraries don't own or provide online access to a book or article you want, log-in to InterLibrary Loan (ILL) to request the book from another library.
Most books available online (for free) are older titles - published before 1923 and in public domain.