For general information on SNMP and writing MIBs, you might find the following books helpful:
Essential SNMP by Douglas R. Mauro and Kevin J. Schmidt, published by O'Reilly and Associates.
Understanding SNMP MIBs by David T. Perkins and Evan McGinnis, published by Prentice Hall.
If you intend to use the Entity MIB for the management of hardware, you should read the following RFC:
Internet Engineering Task Force RFC Number 2737 on the Entity MIB at http://www.ietf.org/rfc/rfc2737.txt.