The following books contain more information about SNMP:
SNMP, SNMPv2, and CMIP: The Practical Guide to Network-Management Standards
by William Stallings
Addison-Wesley Publishing Company, Inc 1993
ISBN 0-201-63331-0
The Simple Book: An Introduction to Management of TCP/IP-based Internets
by Marshall T. Rose
Prentice-Hall, Inc., 1994
ISBN 0-13-177254-6
Internetworking with TCP/IP
Volume 1: Principles, Protocols, and Architecture (third edition)
by Douglas E. Comer
Prentice-Hall, Inc.
ISBN 0-13-468505-9