In the Beatles' songs one can notice some remarkable style changes. To analyse these, however, one first has to solve the problem of the periodization. Yrjö Heinonen and Tuomas Eerola argue the solution lies in dividing the work of the Beatles into twelve recording projects. Using this intermediate level between song and period a more reliable picture of the change of the musical style of the Beatles can be obtained.

See also this page of Beatles recording projects which lists every Beatles song in the order in which they were recorded, with dates. (Beware that funky European day.month.year format, though)

EDIT: also, A flood of flat-sevenths. Or, what are all those flat-sevenths doing in the Beatles' Revolver?