A great map reading resource is published by American Education Publishing. It's "Maps & Geography" - hundreds of pages of map reading worksheets. They're fun and bright, and take the kids through map reading skills on a good gradient approach. Different types of maps are explained.

All my kids have enjoyed this workbook.

Half of the book deals with basics of map skills, and the other half addresses states - generally, 2 pages per state. It's so much fun that it makes it easy to interest the kids in the different states.