As Michael Legat points out in his book 'An Author's Guide to Literary Agents' (Hale £9.99 ISBN 0-7090-5572-2 available from amazon.com):
'Literary agents are now an accepted and respected part of the literary scene.'
Lists of agents can be found in 'The Writer's & Artists' Yearbook' (A&C Black) and The Writer's Handbook (Macmillan). Those marked with an asterisk belong to the Association of Authors' Agents (62 Grafton Way, London W1P 5LD UK), which has a code of practice.
More here (courtesy Author Network)