Larry Trask's textbook of historical linguistics was first published in 1996. In 2007, several years after the untimely death of that great scholar, this revised edition appeared. However, the old first edition is still widely sold, and it can still be used by university courses to great profit. It could even be used by the individual who wants to learn about the subject. My review is based on that first edition, but if I get ahold of the new edition, I will update it accordingly.
In the introduction to the book, Trask himself categorized it among the competition. He felt that it was more demanding than Jeffers and Lehiste (1979), comparable to Lehmann (1992) but with substantially different organization and coverage, and less demanding than Anttila (1988) or Hock (1986), and I agree. Trask's book does assume some previous experience with basic linguistics, but it is clearly directed towards university undergraduates. It covers the usual ground, with recent research like new phonological theories and ergativity taken into account.
One feature that distinguishes Trask's book is the use of Basque in many examples. Though its speakers are securely located in the very Western European country of Spain, Basque is unfortunately left out from books based mainly on Western European languages due to its odd character. Nonetheless, some knowledge of Basque could come in handy for historical linguists--it would have helped me better understand Heine and Kuteva's monograph on the European sprachbund, for example.
Trask's textbook is also unique among the crowd for the attention he gives to long-range hypotheses, fairly remarking that perhaps linguists have missed some connections because of over-specialization, but soundly debunking the work of charlatans like Merrit Ruhlen who think they can reconstruct some kind of "Proto-World". A nice touch is how Trask compares the public's readiness to accept coincidential similarities between, say, Hawaiian and Ancient Greek to the so-called "birthday paradox".
My major complaint with Trask's book is with the production. The typesetting is hard on the eyes, and the binding does not lie flat. All in all, if I had to pick my favourite textbook on the field, it would be Campbell's recent Historical Linguistics, 2nd Edition: An Introduction, where the author's excellent material is presented in a supremely readable manner. Nonetheless, instructors will have to decide themselves what book is best for their course, and Trask is certainly one worth evaluating.