This is probably the best book available to those who want to learn Basque through the English language. It provides a solid foundation to build on and covers the main grammatical points. It does however fail in one main point - the use of the Guipuzkoan dialect. This dialect although probably the most widely spoken differs from the Batua (Unified Basque) being taught in Schools and institutions in the Basque Country. For those who wish to speak Basque, and who speak Spanish, would probably be better off with J.A. Letamendia's Bakarka series. However this does not take anything away from an otherwise excellent book.
Basque is only spoken by about 700,000 people in the world, so there aren't a lot of resources available to help you learn it (unless you speak Spanish or French: since the Basque Country straddles the border of France and Spain there are more books on Basque in those languages). Colloquial Basque though is very good, better than most in the colloquial series of language packs (which are themselves generally above average), and although it won't take you all the way to complete fluency, it will teach you enough to get by, and enough to appreciate how fascinating Basque is. Alan King (who wrote the book with Begotxu Olaizola Elordi) also wrote another good book on Basque, called Basque: A Practical Introduction.
This book does NOT come with cassettes. It seems there are two Colloquial Basque books. One is a course study with cassettes. This one isn't it. There is an order form inside the book (land post) and for a paltry £12.75* you can order the accompanying cassettes. Have you ever tried to learn a language without being able to hear it? (*The publishers reserve the right to change the prices without notice.)