While the list mentioned above does a great job of providing access to books on fly fishing, sometimes one needs to search for oneself!
There are several online collections that can be useful in this case.
OhioLINK E-Book Center- a collection of tens of thousands of full textbooks, primarily reference and academic titles.
Google Book Search- full-text books limited to only those published before 1923 (out of copyright) as well as "preview" content of newer books.
Internet Archive- a warehouse of many different kinds of media, but their e-book collection has many titles of use, again, primarily for those materials that are out of copyright.