Gnumeric strives to mimic the behaviour of other spreadsheets so books which explain how to use these other spreadsheets will probably be useful to help understand Gnumeric.
The OpenOffice.org spreadsheet named Calc is another popular and highly functional spreadsheet which provides users with the freedoms provided by Gnumeric including the freedom to access and use the source code of the program itself. The documentation for Calc may also help explain how to use Gnumeric and can be accessed through the OpenOffice.org web site at http://www.openoffice.org/.
The manuals for other spreadsheets, even those that restrict users freedoms in several ways, may help to explain how to use Gnumeric. Gnumeric attempts explicitly to behave in a manner understandable to users of the Microsoft Excel spreadsheet. Because Excel has a very large number of users, many books have been written explaining how to use that spreadsheet. Any local bookstore should have at least a few of these books which may help answer any questions.