A book is only as reliable as it's sources and in some cases how much information said sources are willing to provide.

I myself am in a fairly recent ancestry book written by a relative of a relative. The book isn't on my own ancestors, so I'm kind of a collateral relative in it.
In it, it states that my Gran and Pop did not have any children of their own (correct) and that they raised 1 child - my mother (incorrect, they raised my mom and aunt).
*Also they fail to mention that my Gran is my mom's first cousin.
It also states that I went to a university. (I actually went to a nearby community college.)
*It also fails to mention my aunt's three children, my cousins.
Ah... family drama.

Mucking up information everywhere.