Where do you go to do research? Public Libraries can unfortunately be under-resourced and are rarely as well equipped as academic libraries. Some academic libraries and research institutes allow you to become a member of a fee, but this can be quite a commitment if you are not sure the academic tome they have is going to be useful to you or not. Or if you just need a small amount of extra information.
There are some good resources on the web, specialist websites and the like – for instance for me I look at the following:
De Re Militari – Medieval Military History website
But often solid researched academic books are the best source of information. But are much too expensive for the individual researcher to acquire. Here are some options I have come across as alternative:
Scribd – individual users can upload texts. Not sure how legal these are always, but seem to be mostly out of copyright material.
Archive.org – does what Google Books should do!
Ebrary – offer an option to access their academic e-book library for individuals. You need to pay $5 as a deposit if you want to copy or print anything, but otherwise it is a fairly good way to get content for the individual researcher.