I am re-reading lots of my bird field guides this year rather than them gather dust on the shelf.
This book was pioneering in its day, and if you can find a copy relatively cheap (there were a number of reprints) it's still well worth having. The illustration by Alan Harris in particular have really stood the test of time. The book also doesn't cover 'unmistakable' birds and focuses on regular British species and some scarce / rare / vagrant birds where there are subtle ID features from a beginner's perspective.
In terms of relevance today (no mention below means they still seem fine):
Divers - before the awareness of Pacific Diver turning up
Cory's / Great Shearwater - before the split of Cory's into Cory's and Scopoli's Shearwater
Grey Geese - before the split of Bean Goose into Taiga and Tundra and doesn't illustrate the latter, which is more similar to Pinkfoot and thus trickier.
Diving ducks - brief treatment of Lesser Scaup as there had only been one record then
Scoters - before records of White-winged and Stejneger's Scoter which would be included now
Harriers - before the increase in Pallid Harrier which are now regular and need considering with Hen and Montagu's
Skuas - good but South Polar Skua may be becoming regular
Large Gulls - before Caspian was a separate species and much more frequent
Rock / Water Pipit - would probably include Buff-bellied Pipit with increase in records
Wagtails - would probably cover Eastern Yellow Wagtail now but treatment of Yellow Wagtail races was low key anyway
Reed / Marsh Warbler - would include Blyth's Reed Warbler now
Hippolais warblers - Booted Warbler would now be split into Booted and Sykes's Warblers
Sylvia warblers - would need to cover the split of Subalpine Warbler into Eastern and Western Subalpine Warblers and Moltoni's Warbler
Phylloscopus warblers - would include both splits Bonelli's Warbler (Eastern and Western) and Iberian Chiffchaff.
Treecreepers - might cover 'Northern' race of Eurasian Treecreeper
Red-backed and Isabelline Shrike - best ignored completely given changes to species
Great Grey Shrike - might include Steppe Grey Shrike
Crossbill - probably wouldn't include Scottish Crossbill which is a disputed taxon
That all said this still means the majority of accounts in the book are still completely relevant and worth referring to alongside more recent guides. I also like the 'notebook' style of the annotations of the artwork, and a shout out for the line 'Correct size assessment is vital: Peregrines Eat Merlins'.
Due an update in the light of recent work (particularly with the large gulls and the crossbill complex) but still an excellent guide to some tricky bird groups.