As someone who lives and works in Latvia, over the years I have accumulated pretty much every book ever devised for learners of the language, including several no longer in print. This is the best, by some margin. For once, the development of the course is logical and practical and the explanations of the numerous grammatical points that English speakers usually struggle with are as simple and concise as possible. I would particularly like to note how vastly superior this course is to the rival "Colloquial Latvian" course which starts confusingly and rapidly degenerates into a bewlindering chaos of grammar. Ironically, "Teach Yourself" is actually far more colloquial than "Colloquial Latvian". The same is true of the accompanying CDs - the speakers on "Teach Yourself..." take the trouble to speak slowly and enunciate clearly at the beginning, whereas the "Colloquial Latvian" offerings are unclear and even feature dialogues that have clearly been spliced together using people in very different studios! Only quibble would be the speed with which some of the vocabularly is introduced, but this pretty much goes with the territory as Latvian is such an unusual language. Overall I have no hesitation in recommending Teach Yourself Latvian. Visu labu!
This is a great book for building up confidence in the language. It moves swiftly from the basic to fairly complex material by the end, but the inclusion of a CD is a vital extra ingredient to help with pronounciation. Only alternative is find a Latvian! Not an easy language if you're used to the mediterranean groupings, but this book will certainly get you on the road. Useful grammar section at the back too... But be aware that the book is often sold without CD.