On our last trip to the homeschool book fair, we found an amazing book for teaching your preschoolers their letters.
For every letter of the alphabet there is a color page and a letter practice page. 
It teaches both upper case and lower case letters. 
But even more important, they are learning through the book of proverbs!
I LOVE this book and you can get it here:
http://www.graceandtruthbooks.com/product/proverbs-for-preschoolers-through-the-proverbs-from-a-to-z
To know wisdom and instruction, to preceive the words of understanding, to receive the instruction of wisdom, Justice, judgment and equity; To give prudence to the simple, to the young man knowledge and discretion-A wise man will hear and increase learning, and a man of understanding will attain wise counsel. Proverbs 1:2-5
The Portion of my Inheritance 