Dr. Seuss's ABC This book has been around for over 50 years!
The pictures are fun and it is a great introduction to beginning letter sounds--in real as well as imaginative words. Read a few pages at a time to make it enjoyable; the length of the book may be daunting for some children.
Chicka Chicka Boom Boom is a popular, rhythmic tale of letter names. As children progress, you can play games and teach the sounds of the letters.