The Handbook of Second Language Acquisition (Blackwell Handbooks in Linguistics)

As a linguist I highly recommend this book. It goes into great detail about the psychology of language learning, and covers many things that I've had to learn by trial and error over the years I've spent in this field. If I had had this book ten or more years ago, it'd have made things a lot easier! A must-have for the bookshelf of any serious linguist.