It is usually not a good idea to start in the middle of a series. There are too many “back pieces of information” you are not privy to. Of course, for the adult reading, it is a good way to gage if the series is good. The first in a series can always be good. It sets the story up, so unless it is just plain bad, there is not a lot to complain about. But if you are picking up the series in the middle and if you can understand book eight in that series, you are doing okay.
Which is what I did with Mysteries According to Humphrey by Betty G. Birney. There are a lot of “well how did that happen?” Yet, there is just enough background for the adult reader to understand it. And in this book, there is enough of its own story that a lot of that background is not necessarily needed. The story is basically Humphrey, a hamster, has a mystery to solve. And while the entire book is “set in the here and now” there is a lot of fantasy (after all hamsters do not write, talk and try to help humans. Right?). But if you suspend reality, you have a Sherlock Holmes-like mystery to solve: What happened to their teacher, Mrs. Brisbane?
I might pick up a couple others in the series and I am interested in the spinoff series starring Og the Frog (also a class pet and Humphreys best friend).