Hathi Trust
For books and periodicals in the public domain (printed before 1928), the Hathi Trust is a reliable source for scanned copies. No login is required to read them onscreen, but if you want to download a PDF of the whole book, log in by selecting UNLV from the institutional access drop-down menu and you'll be redirected to use your ACE account.
Internet Archive
Some books still under copyright can be borrowed from the Internet Archive Open Library. These books cannot be downloaded but can be viewed on screen for one hour. Create a free Internet Archive account to borrow books for one hour viewing sessions. The controlled digital lending model only allows online viewing of corresponding print copies owned by the Open Library, so if the book you want can't be borrowed because it is in use, search to see if another copy has been scanned. Some publishers may require you to use Adobe Digital Editions instead of your browser to view a borrowed book.
WorldCat lets you search the holdings of libraries across the world.
Interlibrary Loans: To request a book that you find listed there, use the UNLV Find Text button and select Request from another library (interlibrary loan). When you log in with your ILLiad account, it will generate a filled-out request form.
Current academic titles
Books published before 1927