Hathi Trust is a large-scale collaborative repository of digital content from research libraries including content digitized via the Google Books project and Internet Archive digitization initiatives, as well as content digitized locally by libraries.
This is the repository that most of the listings in the library catalog will take you too. It is a very good starting point for searching because all of the items listed can be accessed for free by those with a valid Net ID.
Once you have found the item you are looking for, look to the left hand side of the page. This is also the type of page that the libray catalog will bring you to.
It should look like the above. If you click on the "download whole book" link you will be promoted to log in if you have not done so already. On the drop down menu find University of Illinois at Urbana- Champagin and click continue. You will then be taken to a page to enter you Net ID and password. After you do this you will be able to download the book.