Two groups of people write books about how to solve cryptic crosswords. Most are written by professional crossword setters and a few by solvers. On the face of it, the books by solvers ought to be better, because they spend their time doing the thing they're trying to teach you. In practice, it's the books by setters that are better, because:
* To become a setter you must understand cryptic clues well enough to write ones that a newspaper crossword editor is happy to inflict on the public.
* A cryptic crossword tutorial book needs plenty of examples - both individual clues and whole puzzles. A professional setter can use material they've already written or write fresh stuff for the book. The solver must produce his own material or find a way to borrow material from the pros.
* Although setters spend a lot of their time writing clues, most have been or still are solvers too - often very good ones. They need to do some solving just to see what other setters are doing.
Ged O'Connell is a solver. His website says: "Ged O'Connell was born in Bradford and went to university in Leeds before becoming a teacher. He taught English in Spain, Egypt and Finland before moving to Japan over 20 years ago. He now teaches academic writing and critical thinking at a university in Tokyo." Neither this information nor the book itself tells you much about his crossword solving experience, which seems a shame.
Like many 'how to solve' books, most of the text is organised by clue type, telling you what clue types are used, and how to recognise them. The types are divided into "Discrete" and "combination", with the idea that the 'discrete' type is self-contained with characteristics that distinguish it from other types, and the "combination" type consists of components combined in different ways to produce different types of clues. This really means that "combination" includes the types normally called "charade" or "container & contents", with the possibility that the elements involved have been treated, e.g. by being anagrammed or reversed. I can see the logic, but there are holes in the implementation: in particular, some treatments that can be used in this way are only covered as "Discrete" clues. And in this treatment, a hidden reversal is a "discrete" clue, even though it combines two cluing methods - hidden word and reversal. I don't think this grouping works as well as the ones in other books, and it may leave beginners a bit confused.
In each of the clue type chapters, you typically get a verbal explanation, a 'structure diagram', information about 'function words' (= indicators), some exercises (lists of clues to solve), and a sample puzzle. The exercises often give you checking letters to help you, and may have other tips like bold definitions. The system is perfectly sound, but the implementation is sometimes flawed - the structure diagrams in chapters 12 to 15 are drawn in different styles, which makes the differences between the clue types harder to identify. If you look at similar diagrams in the best of the other books, they're in a style that's consistent throughout the book. Several of the sample puzzles at the ends of chapters have grids in the style sometimes seen in language teaching books, with very few checking letters. These are, quite simply, not real crosswords, and the lack of checking letters means there's a risk that the reader can't gain confidence by solving the puzzle, because they can't finish it.
There is some very useful material in the book. The chapters on 'Ignoring Distractions' and 'Developing Strategies' contain good solid practical advice such as thinking of the clue with each individual word stressed in turn, or considering how the clue can be split into two or three parts. This is the kind of material that the books by setters sometimes miss out or understate. And it's only fair to recognise that the previous review is very favourable - so Ged's approach works for some!
Mistakes: p. 9 tells us that the 'key' (i.e. the definition) is always at the beginning or end of a clue, never in the middle. This is true of very nearly all clues, but 'def in the middle' clues can be written, e.g. by putting a def. between two anagram wordplays. The 'Test Crossword' at the end of the book has a misspelled answer, though the clue does at least make the intended spelling crystal clear.
Omissions: Most importantly: enough practice puzzles: only one complete real crossword is included. A book of this size should have ten or so. Coverage of &lit clues - as far as I can tell, these aren't mentioned. Any description of newspaper puzzles other than those in the Sun and Guardian. Coverage of internet resources - some are mentioned, but there are plenty of useful ones that are not. And finally, that key piece of advice: that you need to practice - a lot! - as well as reading books.
Conclusion: There is a competitive market in "How to solve cryptic crosswords" books. There are good ones by former Telegraph crossword editor Val Gilbert, and setters Kindred and Knight, for prices around £5 or £6. The hardback copy of Don Manley's Chambers Crossword Manual sells for a cover price just 4p higher than this book, and gives you about 35 professionally written daily paper cryptics or custom practice puzzles, compared to about a dozen amateur ones. The result is that for anyone shopping around, this book has little chance of appealing as the right one to buy. I don't think the blame for this lies solely with Ged O'Connell. A publisher surely has some duty to check the market and consider whether the book sent to them by an author has a chance to compete.