The highest percentage of learning in a person's life happens during the first few years. But it's not book learning primarily—preschool kids learn to walk, talk, discern shapes and colors, count, draw, etc. by doing those things. At this stage it's more important to play games with them, tell them stories, read to them, and help them commit important information to memory than it is to instruct them formally.
Spending time with their children seems to be an increasingly lost practice among parents. Jobs, church functions, and any number of other obligations separates the family and diminishes the opportunities for gentle instruction. Children ought to be eased into formal learning, and not till they've been encouraged and allowed to develop tactilely.
Many of the books in this section are worktexts or focus on hands-on actvities. Language and Thinking for Young Children by Ruth Beechick and Jeannie Nelson offers benchmarks and goals to achieve with your child. June Oberlander's Slow and Steady Get Me Ready is oriented more toward motor skills and basic knowledge and provides developmental activities for your child's first five years. You really don't need a lot of books at this stage, though—be creative, work with what you have at hand, and always be looking for moments to guide your little ones.