Scottish Gaelic by William Lamb. Lincom
, Languages of the World/Materials series #401. ISBN 3-89586-727-6. This is definitely not a book for the average beginner, but rather something for advanced learners and crazy linguists like me. So, I suggest you have a peek at it before you buy it. If you understand what he's talking about, it is an excellent resource, especially once you start constructing complex sentences. I also found the treatments of verbs enlightening. It doesn't contain much in the way of pronunciation. There's an index at the end of the book that helps you find things. Overall, it crams a lot of information into little space, so it's quite a thin book for a steep price. I personally love it, but it's not for everybody.
A revised edition of this grammar can be found in William Lamb's new book Scottish Gaelic Speech and Writing: Register Variation in an Endangered Language
, Belfast Studies in Language, Culture and Politics #16, Cló Ollscoil na Banríona
, 2008. ISBN 978 0 85389 895 5. I suggest you get this version, since it will come you much cheaper.
The book is available online