The wonderful "Elephant and Piggie" series by Mo Willems.
Today I Will Fly is the first book my almost 6 year old read entirely on his own. Elephant and Piggie books are accessible even to many 5 and 6 year olds and are a fabulous way to ingrain the pleasure of independent reading. The stories are so silly, with easy vocabulary and spare but charming illustrations. The kids have so much fun, they forget they are reading.
Luke on the Loose
For readers at 2nd grade level and up, the oblivious misadventures of a boy lost in the big city as he chases pigeons in the park. For fun, comic fans can spot Tintin, Captain Haddock, Olive Oyl, and the Incredible Hulk in the book.
Binky the Space Cat
The wacky adventures of a very indoor cat who thinks Outer Space starts in his front yard. For ages 7 and up. Adults will especially enjoy this one, particularly if they are cat lovers.
Rick and Rack books from Balloon Toons
The timeless encounter between unlikely friends: the optimist and the pessimist. Lots of wackiness and a character lesson about the merits of optimism to boot.