I have mixed feelings about this trilogy. On the positive side, there is a lot of math for middle school readers, with each book focusing on different mathematical topics. The first book covers graphing/charts/patterns, the second fractions, and the third equations. For a series aimed at pre-high school readers, these books contain a lot of math.
That said, this series does not really have a story. The "plot" is based on the idea of a seventh grade student named Lennie Miller who solves math problems for supernatural creatures that live in a mystical realm. These creatures are bad at math and need Lennie's help in solving a wide range of problems.
Unfortunately, the series comes across as a collection of math problems rather than a unified narrative. While the author absolutely gets the "Math" part right in this trilogy, the "Fiction" part is convoluted at best. Although I do not want to discourage people from reading these books, especially given their strong math content, in my view there are better books for young readers interested in math fiction.