Considering the enormous variety that can be found, your best approach is going to be looking for a portal or respected program that has collected resources on a particular topic. Below are a few places you might consider starting.
Depending on your research topic, keep in mind most nations and states have official libraries. These libraries are usually digitizing some of there resources, so it is worth looking at these sites to see if they have undertaken any digitization that may address your topic.
Yes Google Books - Since they undertook a large scanning initiative based on collections at various research institutions you may find some vaaluable resources.
Look at mulitple engines when doing research.
You know the regular ones, but also
You must evaluate all sources, but you have to be particularly careful when looking at free online sources.
Paticularly, with online research you need to look at Who creatd the page. What is there authority and then why did they post it. Well designed sites will tell you all of this upfront. Unfortunately, sometimes you may want to use a site that isn't well designed. One tool is to look at where the site is registered.
Faganfinder offers a URL analysis tool. Look at the Globalwhois and the Whois tabs to try and determine the creator. If you still can't figure it out, try and email the site for more information.