Well my kids have enjoyed the following--

Grolier's I wonder why series(set of 24 books) (when Senior was 2).Senior was a walking encyclopaedia by 3.
Story a Day
Time Life Early Learning Program(set of 15)
Time Life Value series(set of 15)

Junior uses them now while Senior has moved on to
Grolier's New Book of Knowledge (set of 20 books) and the
The Grolier Science Encyclopaedia(set of 10 books)

They provide an amazing range of facts which keeps my kids delighted.
Even the Young Discoverer Series has a good range of books on various topics.
Happy Reading ...

Twiceblessed