As well as searching on Primo, try searching in some of the e-book collections. All of these e-books are on Primo but the search functions in the collections can delve deeper into the books held in those collections.
Historical Texts - EEBO, ECCO and more
The Library subscribes to JISC's Historical Texts which includes Early English Books Online, Eighteenth Century Collections Online and Nineteenth Century first editions from the British Library.
This short film will tell you a bit more about it:
Some publishers (particularly some US university presses) publish freely available open access versions of some of their catalogue of secondary and critical material:
There are now many collections of books that are now out of copyright that are made freely available online:
Online reference books