Just because I’m compulsive, this full listing, in chronological order, of the series (all books by Sue Townsend):
The Secret Diary of Adrian Mole Aged 13 3/4 (1982)
The Growing Pains of Adrian Mole (also 1982)
True Confessions of Adrian Albert Mole, Margaret Hilda Roberts and Susan Lilian Townsend (1990)
Adrian Mole: The Wilderness Years (1994)
Adrian Mole: The Cappucino Years (1999)
The second book seem to be available in the USA only in an omnibus of the first two entitled The Adrian Mole Diaries. The rough UK equivalent is Adrian Mole: From Minor to Major, which also includes some material from True Confessions.
True Confessions and Wilderness Years seem to only be available in the USA in an omnibus volume (which might include material not in True Confessions, but which also seems not to include the other characters from that book) called Adrian Mole: The Lost Years
And that seems to be where we are to date (not including TV tie-ins, playscript versions, and other oddities.) Amazon.co.uk lists as coming in October of this year a book called Adrian Mole: A Comic Novel (I assume that subtitle is just a tentative placeholder).
Anyway, on to personal thoughts. I’ve read the American versions of the earlier books (Diaries and Lost Years), but not yet Cappucino. I liked the two books in Diaries quite a lot when I read them – I thought Adrian seemed real and human. (I’m close to his age, but I only read the Diaries in the mid-90s, so I didn’t have much direct identification with Adrian (though I’m pretty sure I would have if I’d read them in the early '80s).
To encapsulate: the first two are wonderful looks at the world through the eyes of a particular kind of smart but unworldly kid, and the tone is perfect. Anyone who was a smart, weird teenager in the early '80s will probably love them.
Lost Years, which I just read last month, I had more mixed feelings about. The True Confessions material takes a scattershot approach, and drags Adrian from young adolescence and the early '80s into young manhood and the Gulf War. Unfortunately, it also take a lovable “might-be” into an obnoxious loser. He keeps the same personality traits, but they’re much less endearing when he’s 20 – I started to feel, part-way through, that I was meant to be laughing at Adrian rather than with him. All sorts of unpleasant things happen to him, and he doesn’t seem to learn anything – he seems to turn into a rather dim adult from what was supposedly a bright teenager.
He does grow up a bit by the end of Wilderness, but there’s some tough slogging in the middle. There were things I laughed at and felt horrible for laughing at, as if I was personally making Adrian that miserable and unhappy. It’s the sign of a pretty good writer, sure, but that’s not the reaction you want to get from a light comic novel. All in all, I’d say to stick with the first two.