Quick Search is the tool to use for finding books owned by the ISU Library. It's located on the ISU e-Library home page. You can use the "advanced search" feature to get to an interface where you can select which "field" (author, title, subject) you'd like to search in (although you don't have to restrict your search to those fields). You can also used this advanced interface to select the type of materials you're searching for (you might want to select "books"!) and specific publication years.
Online books can be found along with print books doing the same type of search as mentioned above.
The subtitle of the sixth edition of this book, now in electronic form, says it all: "An Annotated Listing of Reference Sources in English Literary Studies." Author James L. Harner has provided an exhaustive list of reference resources in all aspects of English literature studies. There are over 1000 annotated entries in this book that discuss over 1600 different books that cover topics from early English to 21st Century literature, poetry to Children's literature, and authors writing in English all over the world. This, along with the MLA International Bibliography database, should be the first resource consulted when starting research. The book can be accessed here.
WorldCat is a database that serves as a catalog for most of the holdings of the academic libraries in the world. Use this to find books on your topic not owned by ISU. You can then, in WorldCat, easily request a book through the ISU Library's Interlibrary Loan Department. WorldCat can be accessed by clicking on the Article Indexes and Databases link on the Library home page, and then selecting "W" from the alphabetic menu. Or click below to go to WorldCat.