These are my very favorite nature books that focus on letters, shapes, and colors. Because they center around traditional, developmentally age-appropriate preschooler topics, these books make welcome additions to most kidlet libraries (even in those families & schools where nature is not considered a high priority.)
Discovering Nature's Alphabet
Waldorf Alphabet Book
Star in my Orange (Looking for Nature's Shapes)
Looking at Nature: What Shape is it?
What Color is Nature?
Planting a Rainbow