The Beatles Sesssions book is a great historical read for engineers. Session notes, equipment, numbers of takes, track sheets...for every single session the Beatles ever did anywhere. Martin's book is good as well.
In 1961-62, Martin had been trying to talk EMI into buying a bunch of 3m 1" 3-tracks as used here in all the LA studios such as Capitol. EMI kept stalling him off and finally, when they did update the existing old 2-tracks, EMI decided to go with a hybrid, company-built set of 1" 4-tracks. Which was easy for EMI because they built so much of their own stuff anyway.
As soon as the bunch of 4-tracks arrived, Martin planted two of them in the control room of Abbey 2.
The procedure for the next 5 or so years was this...fill 4 tracks of machine 1..submix to 2 tracks of machine 2..add 2 tracks..submix back to 2 tracks of machine 1. etc etc. Which is how it comes to be that there are hundreds of reels of 1" Beatle tapes in the EMI vault.
When all those tapes and 1" machines were hauled out of storage and the "non" submixed tape tracks were transferred to Protools a few years back and remixed, one finds that early-mid-late Beatle recordings are actually 30+ tracks of individual things. Not the popular folklore that the Beatles did everything on the confines of four tracks and four tracks alone. Not true...They didn't even do that in 61-62 when they were recording in stereo..even then, the setup was to ping-pong and submix between two stereo machines.
As far as the twin four-track years..."Penny Lane" alone is a 36-38 track song...and sounds so incredibly beautiful on the new stereo/surround remix on the dvd anthology. Gone are all the limitations imposed by those original 2 4-track machines.
The Sessions book is great also because it shows how incredibly frustrating it was for all the guys involved in the sessions to be working with the limits of the tape...sessions literally had to simply stop whenever it was time to do a submix ("reduction mix").