Ultra's already mentioned Moomintroll, which I had forgotten earlier, to my embarrassment.

I would add Ogden Nash's book, Custard the Dragon, and to a lesser extent its sequel.

Shel Silverstein's books, in particular The Missing Piece and The Missing Piece Meets the Big O, are a huge favorite of my DD7 and have been for years.

Jack Prelutsky's A Pizza the Size of the Sun is another grand silly poetry book that is very popular with both my kids.

The Lyle the Crocodile books were my absolute favorites when I was very small.