The intermediate books are not essential. When a student finishes Jelly Beans, they have all the arithmetic they need to start Fractions, but because of how quickly students can progress in the elementary books, a student might not be old enough to really be ready for Fractions (author recommends 5th grade). Kidneys, Liver and Mineshaft allow for more practice of arithmetic while the student gets older.
I love the idea of weaving math and information about a variety of subjects into a silly story. I do not love the negativity of the Fred stories. I have a very different world view than the one that is presented by the author, and I’m starting to wonder if it really was a good idea to suggest his books to my kid in the first place. I have definitely been discussing with my kid that it is not ok to not eat. That it is not ok to expect someone to eat less/differently in order to make someone else like them (different book in series). That it is not ok to suggest that someone should change their appearance to make someone else like them (different book in series)... I’m probably not going to get any more books from this author unless my kid specifically requests them. And I’m not going to make the mistake I did with some of the other books in this series, of not first reading them myself. If my kid asks to read any more books by this author, I will read them myself and we will discuss them afterward together.
From a middle schooler/highschooler (homeschool😭) using this for math I think it was really good! Easy to understand and helped memorize math facts a lot - I've read all of them from apples to this.