One feeling I've had in response to reading Ian MacDonald's Revolution in the Head is a pleasant sort of shame. Whether you agree with his conclusions or not, it's undeniable that MacDonald thoroughly knows The Beatles' catalogue, and what this can lead to is him essentially telling you, on grounds or in terms you hadn't previously considered, why you like a particular song or why it's so compelling. You might adore "It Won't Be Long" and have well thought-out reasons for doing so, but MacDonald will supply you with better, more incisive ones. It can leave you a bit embarrassed, but you should also welcome the new perspective.
A notable example comes in the section on "From Me to You." MacDonald writes: Like most of Lennon and McCartney's few recorded full fifty-fifty collaborations, FROM ME TO YOU proceeds in the two-bar phrases a pair of writers typically adopt when tentatively ad-libbing at each other. The usual result of such a synthetic process, in which neither contributor is free to develop the melody-line in his normal way, is a competition to produce surprising developments of the initial idea. As in I WANT TO HOLD YOUR HAND, the variation surprise in FROM ME TO YOU consists of a sudden falsetto octave leap, a motif first tried on the chorus of PLEASE PLEASE ME (itself rewritten in this to-and-fro fashion) (pg. 78).
Let me first say that I've long considered "From Me to You" one of the Fabs' strongest early-period songs. When I wrote about it here, I highlighted some of the parts that MacDonald emphasized, like the falsetto ("The breezy joy they were undoubtedly experiencing reveals itself throughout the song. It's in the high notes that John reaches for on the harmony"). In short, I felt I knew exactly why I appreciated the song so much. But then I read MacDonald's take and found out differently. He nails it: it's not just the mere inclusion of the giddy falsetto that makes the song; it's the "surprise" of it, the way it seems to come out of nowhere.
The author elaborates: Yet where the Americans built falsetto into their four-part harmony, The Beatles wielded it as an isolated device, and it was mainly these sudden hair-raising wails that made their early records so rivetingly strange (pg. 79).
MacDonald makes it all seem so obvious.