To rename a custom field, open the data entry window, right-click the data field and choose Rename Field from the popup menu. You can also rename fields from the 'Database Setup' window (Tools=>Database Setup).
The Book table contains the following custom fields:
Text
Custom 01, Custom 02, Custom 03, Custom 04
Lookup
Custom 05, Custom 06, Custom 07, Custom 08
Multivalue lookup
Custom 19
Date
Custom 09, Custom 20
Numeric
Custom 10, Custom 21
Checkbox
Custom 11, Custom 12, Custom 13, Custom 14, Custom 15, Custom 16, Custom 17, Custom 18, Custom 22, Custom 23, Custom 24, Custom 25