The "Key to..." series from Key Curriculum Press is a set of workbooks that teach you the concepts as you answer questions. As a teacher, these workbooks were a real lifesaver when I had students that needed to work on their own. The series available are Key to Fractions®, Key to Decimals®, Key to Percents®, Key to Algebra®, Key to Measurement®, Key to Metric Measurement® and Key to Geometry®. Each set of lessons starts with very simple concepts and slowly increases in difficulty until the concept is mastered. The nice thing is each workbook is only a couple dollars each and you can buy an individual workbook or the entire set.

For Pre-algebra I'd recommend the entire set of Key to Fractions® and from Key to Algebra® the workbooks Book 1: Operations on Integers, Book 2: Variables, Terms, and Expressions and possibly Book 3: Equations. This should give any student a good foundation in Pre-algebra and a good introduction to the basics of Algebra.

Hope this helps,

Janice