I've only briefly perused
http://www.ef.edu/english-resources/english-grammar/, but it looks decent enough.
It really depends what you're after. If you want a comprehensive description from a linguistic point-of-view then, yes, Amazon (or your preferred online bookseller) is the place to go. But if you just need some rules of thumb to help you make fewer mistakes, some of the instructional grammars available online might be adequate.
"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons