There is not much potentially concerning here.
The prologue opens with a poem that says it is from a “Holy Book”, but is an odd section of wicked ranting from the villain of the story (mostly about wanting to take over). It is never over the top, but may be a confusing start for a young reader as there is no story context behind it at this point.
The book is light throughout, but there are sections with violence and villains that could be disturbing for the youngest or most sensitive of readers. Mom is also very sick and is not seen to have much chance to live, which may be a heavier topic for some.
Below is a list of a handful of additional items to be aware of for your reader:
Boys exchange minor insults on the baseball field.
Halloween is mentioned in a positive/fun light.
The “witching hour” is mentioned in one section several times (referring to 3 in the morning)
There is a reference to the “Mothman”, which the story describes as a real scare from the 1960s and uses to explain one of the villains (one that does not come across as a “horror” overly scary dark being)
The main villain is said to have “enhanced himself by many dark spells and pride-hearted techniques”. There is a later comment that “he took his power from all the realms and is like a god, or Hercules or something”.
Something is described as “a god-like being”.
There are a few song references, all from the 80s (e.g., “Our House” by Madness)
One character reads comics, which are referenced in a positive light, but today are typically very dark.
An older woman smokes a pipe. At the sight of a stumbling animal Jack jokingly says that it “looks like he had something other than water to drink”.
