I can't be 100% sure, but the rather glowing 5-star review from "jackie", which was the only other review here when I wrote this, looks suspiciously like a review from the publisher. The note from the publisher at the end of the book is signed "Jackie Gray".
I'm sorry to be negative about a book by someone who has produced many sparkling clues in the Azed clue-writing contest, but this book is not what it might have been. Its main big idea is that if you know what kinds of cryptic clue there are, recognising them will help you to solve cryptic crosswords better. It certainly helps, but I'm not convinced that it's the "essential pre-requisite to success" as the back cover states. And like some other "how-to-solve" books from small publishing companies, this one demonstrates that unless the author is really strict with himself or gets expert friends to read the text, the lack of strong editing means that significant flaws are not spotted.
The book is short, and there's no harm in that, if the content is good. But the presentation of clue types is somewhat confusing, and some sample clues are questionable or seem inappropriate. We start off with anagrams - handled well except for the set of sample clues which has no anagrams of more than 6 letters. I guess this is to keep them easy, but in real puzzles, anagrams are quite often used for long answers, and not getting any practice at these seems a mistake. Hidden words come next - mostly fine but we get a final misleading clue using an obscure plural for which you need Chambers dictionary - fair game in a barred-grid difficult cryptic but not in a book which later recommends Daily Telegraph puzzles as a good starting point. At Chapter 3 it all starts to go a bit wrong. This chapter is about abbreviations. As Dexter says, "abbreviation" is not a clue type but abbreviations abound in cryptic clues. Fair enough, but most of the sample clues in the text are charade clues. Given the "big idea" already mentioned, it would surely make more sense to deal with this clue-type first. The next chapter is about double definition clues, and describes them well enough but then Dexter warns us that the sample clues are not his best bunch. He's not kidding - of the twelve, I reckon four rely on incorrect definitions and are therefore dud clues, and there are also obscure defs used. Example: "Brass elephant" as a clue for TRUMPETER. As any dictionary will confirm, "brass" decribes a group of orchestral players, not an individual, except in the phrase "brass player". Skipping on to the final clue type covered, the impression is given that &lit clues only exist as anagram-based clues. This chapter has the only mention of &lits in the book - even though there are other &lits or near-&lits in many of the other examples. (And the standard structure for many clue types - definition and separate "wordplay" - never gets properly explained. This (along with the times when it doesn't apply) is surely as important for beginners to understand as the range of types.
At the end we get a crossword by Dexter, based on Morse and Lewis. I can understand his desire to include his own characters, but the effect is that the book's only complete puzzle is one with a theme. The fact that this is quite unusual in some newspapers goes unstated. As the puzzle relies on knowledge of Morse, including book titles, and contains clues which Dexter freely admits are poor, it's a pretty hopeless choice as a single sample puzzle. And some clues are explained in an unhelpful way - such as the subtle anagram clue for an author's name, whose "anagram fodder" appears with no information about where it came from.
Finally, of the 104pp in the book, about 21 are blank or unnecessary - whole page chapter headings or blank pages for notes, at the ends of chapters that happen to have an odd number of pages.
Conclusion: unless there's something about Colin Dexter's treatment that the rivals can't provide (and that seems to boil down to the Morse connection), the books by Don Manley and Tim Moorey still lead the field, and this ranks with the also-rans.