I've been a little hesitant to comment on this thread so far as the last time I posted some comments on a field guide,
it all got just a little bit out of hand...
However, I'm going to chip in with a few comments as, of the three main aspects of this book which disappointed me, only one (the maps) has been mentioned so far. Like others I'm surprised at just how many of the maps contain errors - generally not huge errors, although no Wessex population of Montagu's Harriers, and Balearic Shearwater apparently not visiting Cornwall seem like the sort of error that would have been picked up in a proof-reading browse by even a relatively inexperienced birder; given the calibre of the team involved in this book, I really don't know what's gone wrong here and can only assume this task was delegated to someone not in the authors list, and they ran out of time to check the results.
The other two areas where the book feels incomplete relate to the selection of photographs, and in some cases, the selection of species. If I was to compile a list of the photographs that I would want to include in a guide like this, I would try to think of all the possible taxa that I'd want to include (residents, migrants, vagrants, naturalised introductions, escapes, subspecies, hybrid etc) and then for each of these, compile a list of the plumages that I'd want to illustrate, in which positions and from which angles, and then take this complete list and rank it into priority order, based on the likelihood with which a birder is likely to encounter this species/plumage/pose, and use that as the basis for my photo search. Clearly the authors have done something very similar to this, but there are some odd omissions.
Firstly, juvenile plumages get a raw deal. There are some common, widespread and easy-to-encounter and photograph juvenile plumages which are not shown (e.g. Pied Wagtail, Wren, Chaffinch, Coal Tit) as well as some scarcer species where juveniles differ enough from adults to have made it worthwhile making a special effort to seek out a photo (e.g. Chough, Ring Ouzel and Lesser Redpoll). Some first-winters are miscaptioned as juveniles e.g. Spotted Flycatcher - where only tiny traces of juvenile plumage are left, and Yellow Wagtail - the individual captioned as juvenile/first-winter is definitely the latter. In addition, juveniles of Northern Wheatear, Stonechat, Common Redstart, and Nightingale are hidden away on separate pages from adults of these species, with a reference to this page buried in the species text, or not mentioned at all in the case of Stonechat.
Secondly, why are so few hybrids, escapes and aberrant plumages shown? To give a couple of examples: (1) I can count the number of (presumed) vagrant ducks I've found on the fingers of one hand, but the number of escaped and hybrid wildfowl I've just stumbled across while casually checking through flocks of commoner species runs well into triple figures. Their prominence in the book should surely reflect these sorts of likelihood. Some of these are shown, though often in just one plumage, and many taxa that the average birder is much more likely to encounter than they are to find a Barrow's Goldeneye, say, are omitted entirely. The most obvious example is the complete lack of a Muscovy Duck anywhere in the book - or is the reference on page 551 to this species attacking one of the authors the cause of this (safe spaces and all that) ? (2) I estimate at least 1 in 20 of the Carrion Crows in my home city are of the white-winged type, but this form isn't depicted anywhere.
These gaps are not by any means unique to this guide - Collins suffers in the same way to an extent - but given the care and attention to detail evident in so many other aspects of the book, they make it look unfinished. If I was asked whether this book was worth buying, I would probably on balance say yes - the book is distinctive enough and the problems not significant enough to make it a dud - and the work required to fix them is large enough that we're not in "just wait for a reprint" territory as we were with Collins or the second edition of the Sibley guide.
