The copyright clause determines who shall have the right to license copies of a book.
In the USA, copyright is created automatically upon creation of a work of art or writing, and copyright is vested in the creator of the work, who may license to others the right to make copies of the work.
For
most works of art (including books), the copyright
holder is the creator (author), but for many
series guidebooks, copyright is held by the
publisher.
To some extent, this is understandable because
of the nature of guidebooks. Often they are
collaborative efforts, in which case it would
be unweildy for four or five authors to share
copyright.
Many
times a guide is originally written by an
author or authors, but revised by others.
A contract which gives the publisher copyright
is usually termed a "work-for-hire" contract,
because the author is working as a temporary/contract
employee; he or she is not considered to be
creating the work alone.
Having copyright in a work does not mean
you "own" it, though.
The other provisions of the contract may give
or take away far more valuable rights than copyright.
It is not uncommon for publishers to offer contracts
that allow the author to retain copyright,
but include clauses that take all
other rights forever, rendering the author's
copyright valueless.
It's always good to get copyright if you can, but its value depends on the rights you retain after you've agreed to allow a publisher to publish your work.