Larry Trask was one of the greatest contemporary writers on linguistics and grammar, and all his books are packed with lucid, complete, and often very witty descriptions. The cover of this book claims that it shows "How to use English correctly". This is perhaps misleading, because the book is not principally a usage guide. (Two other books by Trask - "Mind the Gaffe" and "The Penguin Guide to Punctuation - do provide solid, clear guidance on writing; anyone looking for instruction on how to write well should get hold of copies of these books first - they are indispensable.) The "Dictionary of English Grammar" contributes to better writing in a more indirect way. It provides succinct and clear definitions of terms and concepts, many of them quite technical, that are used to describe English grammar. In so doing it "enables us to see how language works and how to use it in the right ways" (from the back cover). Despite running to only 148 pages, the book is pretty comprehensive, and just as important it is contemporary. Authorities on English grammar often use terms and concepts in different and conflicting ways, and this can leave the student perplexed and irritated. A major value of Trask's book is that it summarizes these differences; for example, traditional and more recent definitions of "clause" are both explained. So if you have been confused by the varied terminologies used in different sources, this book will provide invaluable help in resolving your questions. This book is a handy reference for anyone interested in the structure of English and the vocabulary used to describe it. But be warned; this is a dictionary of grammatical terms, not a grammar. It makes an ideal partner to a student grammar (such as Longman), but it is not a substitute for one.
Incidentally, the earlier reviewer who was made cross by the use of "who" in an object position would do well to read the entry under "Objective" on page 92, and better still should look up the entry under "Whom" in "Mind the Gaffe" (p296).