I too adored Tom's Midnight Garden. I visited the garden where one of the film adaptations was made (Chenies Manor, Buckinghamshire).
There were also the "Roundy and Squary" books by Roger Hargreaves. These were nowhere near as well-known as the Mr Men books, but they were very funny, contained a lot of human truth, and probably summed up Mumsnet. Here are some lines from them (they can be found on Youtube):
"Roundies love to play in the grass. Squaries think grass is a nuisance." (Picture of squary with lawn mower)
"Roundies whistle while they work. Squaries are far too busy working."
"Roundies love to play in the rain. Squaries hate getting their feet wet."
"Roundies can laugh at themselves. Squaries can only laugh at other people."
"Roundies understand squaries. Squaries don't even understand other squaries."
"Roundies go out for long walks. Squaries go in for long faces."
Phoebe and the Hot Water Bottles: now extremely rare and out of print. It was a beautiful book, and unusually for the time, featured a child with a single father; but it was discredited for the idea of giving children puppies as presents, and trying to put out a fire by yourself.
Also, one of the less well-known Roald Dahl books: The Wonderful Story of Henry Sugar. In the same book was the brutal short story "The Swan", which I loved at the age of eight, in spite of its nastiness, in which two ruffian boys terrorise a younger middle-class boy, which makes for very uncomfortable reading now, and indeed, some parents tried to get the book banned. (Sorry to derail a lovely thread with that.)