I've now had time to peruse the volumes, and, as others have suggested, they're a tour de force. The volume of information presented is outstanding and there can be few if any birders who won't learn a huge amount from their pages. I'd go so far as to say that, the odd birding genius aside, readers will learn something new on pretty much every page. There's no doubting the huge authority vested in these books and it would be an act of great temerity for a mediocre birder like me to level any serious criticism of the volumes. However, I am up for a bit of petty niggling!
1 - the lack of behavioural traits useful in identification (tail dipping, etc)
2 - the entire absence of notes on vocalisations. This becomes particularly irritating when, for example, it writes (of Iberian Chiffchaff) "In the field certain ID without vocalisations is difficult" but doesn't give you any clue what those vocalisations may be! Ideally, every text should have a QR code linking to a sound file (as other books are starting to do) or at the very least some sort of descriptive note where vocalisations are noted as critical for ID
3 - that annoying white strip at the foot of every page (I said I was being petty) .... vocalisations could go there!
4 - absence of maps. I understand why they were omitted but I wish they hadn't been!
5 - judicious use of bold/italic type to pull out vital words would be helpful.
6 - following the recent trend by using a "throw-the-book-out of-the-window" index where the alphabetical order is determined by the first word of their English name (so you need to know it's now Mediterranean Short-toed Lark, not Lesser Short-toed Lark and Penduline Tit is Eurasian Penduline Tit to find out where they may be found). To be fair, inside the front cover there's a pictorial family index but that's no good for those of us who instinctively turn to the index at the back.
7 - as these books are not going to be lugged out into the field then a larger format (a la 'The Handbook of Western Palearctic Birds) would have allowed for a more readable typeface for older birders like me (and that's most of us in the UK these days!). A slipcase would have been nice too!
So far so petty, so I'll reiterate that these two volumes are superb and fully deserve all the plaudits coming their way when they're reviewed in the birding press.