Ask your child's teacher or principal to recommend titles or book series. Ask other educators like guidance counselors or special education teachers if your child is having difficulty in school.
Talk to other parents especially those with slightly older children. Find out what books they bought and why certain ones were good.
Read book reviews or information on the Internet about specific workbooks. Go to parent websites to see what other parents think of certain books.
Visit local teaching stores or shop online for teaching resources. These stores carry workbooks in many subject areas for parents, students and teachers.
Check the curriculum to see what your child should be learning. You can check with your child's school or read the curriculum requirements online. Buy books that reinforce the skills outlined in the curriculum.
Make sure you use workbooks that follow the standards of your country. If your country uses the Imperial math system, you don't want to buy books using metric.
Check the prices. Sometimes large stores or online stores sell books at discounted prices. Check second hand stores for nearly new books.