The AWS Standard A2.4 is the "standard" and is very easy to use as a study resource. The AWS welding Handbook Volume 1 8th and 9th edition are useful also. The welding handbooks also contain much more information including AWS standard terms and definitions.
Hobart Institute of Welding Technology offers a "Symbols for Welding" course material based on the AWS A2.4 standard. For further information see http://www.welding.org or call 1-800-332-9448, ext. 5433.