A is for Apple, is a great book for teaching the children at all different levels. It is a must have.
Here are some of the ABC activities I am using with my little ones. They are mainly hands on activities because I only have a few minutes with them so I do not like to use worksheets. Also I believe little ones learn more when they are having fun.
I also purchased this unit, Rice Box and Cookie Sheet ABC Center Pack. It is fabulous. I am using it with my four and five years old. I purchased my magnetic letters at Lakeshore Learning. There are several great activities.