If preschool age (3-5), are you looking for:

1) Story books
The PB Bear Series
Fiction

2)Tangible books (such as touch and feel, scratch and sniff or board books?)
Touch and Feel series
Scratch and sniff series
Board Books

3) Early Learning books

4) Books about a particular topic, such as myths and legends or prehistoric earth?
Animals
The My First series
Myths and Legends
Nature
Pets
Prehistoric earth
Religion
Special Events
Transportation

5) Activity Books