While the material in these books is lots of fun, this is not a book that should be left lying around where younger readers to get their hands on it.
There are various entries in unexpected parts of the books that touch on sexual acts, some of which are decidedly outside social norms (including references to urine and "water sports", insects and formicophilia); same-sex relations (both positive and negative statements); genitalia; fellatio; and more.
For kids in high school, this may be acceptable, depending on the maturity of the child, but this is certainly not a book that should be left where children in the lower grades can read it without supervision and adult guidance.