I like using workbooks for my son (who’s currently in preschool). It gives a structured sequence of learning material, with fun stickers and colorful pages to make learning more engaging. I suggest just typing in “(current grade, or one or two grades higher) workbook.” I love the tracing books where you can keep wiping them off and reusing them. It’s also good to get a variety of books, such as books pertaining to math, rhyming words, STEM, etc.