The Pebble Plus series from Capstone is one of my favorites. A simple text designed for beginning readers makes the text easy to share with preschoolers and the big format and bright photos in each book make them very visually appealing. More sets are coming out all the time and they cover high-interest topics like sea creatures, vehicles, dinosaurs, and many more.
The About... series by Cathryn Sill and illustrated by John Sill is another great series with simple text and really beautiful illustrations. Each spread has text on the left page and a full-page painted illustration on the right. The text is great for a general overview and additional information is included in the back of the book. Books in this series cover many different types of animals and habitats.
The Let's Read and Find Out Science books are a great choice for science topics. For preschoolers, look for Stage 1. Many areas of science are covered including the human body, weather, animals, plants, and more!
Last, but certainly not least, I want to point you to a great list compiled by preschool math resource teacher Judy Ballweg at the Madison Metropolitan School District. Her list of Books to Nurture Mathematical Thinking (link opens a pdf!) is a great resource when you're looking for math books for little ones. Her list is one of many good ones over at MMSD's site. It's certainly been helpful to me this summer!
These are some of my favorites nonfiction series and resources for preschoolers. Be sure and check out the Non-Fiction Monday roundup over at Anastasia's blog!