Don't be too trusting of Google - it can only tell you about the info that somebody went to the trouble of putting into a web page. I've been comparing Google results with books from the library lately, and there is a lot of encyclopedic information that is simply not online, or not being indexed. [...]
I would like to second this. Google is a great tool, but everything is not on the web, and everything that is on the web is not indexed.
Historical data is one gaping hole, such as the history of a town or small city, or a biography of a person who is no longer prominent.
There is also relatively little coverage of past controversies. You can still find articles about the refusal of Minnesota's state government to fund construction of a new stadium. But what can you find about the controversy that went on regarding the construction of the present stadium, 20-odd years ago?
And some corporations maintain official web silence on controversies they are involved in. Continuing an earlier example, Monsanto doesn't have a rebuttal of the Ericson & Hardell study on their web site, but if you write to them and explain that you're writing an article about the safety of glyphosate, they'll send it to you, with references.
Also, the web is a terrible place to try to get a sense of the relative importance of something. Books aren't perfect but you can get a rough sense of importance of, say, composers or painters by reading through a survey of the era in which they were prominent.
Louis