Essential Portuguese Grammar (Dover Language Guides Essential Grammar)
This is a great reference for Portuguese grammar. My only reasons for not giving it 5 stars are that it is a little dates (published in 1966, and a couple formal changes to the language have occurred since then) and the layout is not as clean as it could be. The latter is more an issue when one is flipping around looking for something - the poor layout makes it easier to miss what you're looking for.
