One of my favourites for the layman is
Architecture: the Natural and the Manmade by Vincent Scully. It's very readable and very well illustrated, and is available used on Chapters.
A good pocket field guide is
American Architecture Since 1780: a Guide to the Styles by Marcus Whiffen, although he weirdly calls Art Deco "Modernistic". Also available used on Chapters.
The definitive text for architecture wonks is probably
Architecture: Nineteenth and Twentieth Centuries by Henry Russell Hitchcock, also used on Chapters.
The indispensable resource for architecture keeners as far as I'm concerned, is
The Penguin Dictionary of Architecture by Nikolaus Pevsner, John Fleming, and Hugh Honour. An encyclopedia of architects, styles, and terms. Soon you'll be using words like 'fenestration' and boring people at cocktail parties. I keep it by my bedside in case I forget what a lesbian cymantium is. (It's a cyma reversa with a leaf and dart motif! Honestly, you people.

) Available used on Amazon.
Have fun!