Actually, I'm not sure they're at all good...but they fall in the category of "I have good memories of reading this as a child, tracked it down, and will now recommend it to people".
If that's not a ringing endorsement, I don't know what is.
Here's a ringing endorsement, though, courtesy of my good friend and serial commentator with a crush on the captcha - you may know her as Lannis): The Book Depository. Free shipping, anywhere in the world they ship to.
Free.
Free!
I'm calming down.

The first book for this category is The Jolly Postman series, of which there are three: The Jolly PostmanI'm going to go out on a shaky limb here and say that the quality of the writing in any of them is so beside the point that the point is allowed to wear an "I'm With Stupid" t-shirt and you still have to buy the books.
These books have envelopes in them. With stuff in the envelopes. Stuff. In. The. Envelopes. Like a Humpty-Dumpty puzzle, a postcard from Cinderella's honeymoon, a birthday card for Goldilocks with a pound note in it, and an advertising circular for witches...among other awesome things.You can't get these out of the library. Even if you could, some dumb kid (someone else's, obviously), will have taken all the stuff out, messed it up, and put it back in the wrong envelopes.
My only problem is deciding when Miss will be old enough to read them without wrecking them.
I figure I'll be safe when she's around twenty-five.