These books are only a few of my favorites-- I highly recommend visiting Sea Challengers and using their site search engine to browse their selections!

1001 Nudibranchs - Catalogue of Indo-Pacific Sea Slugs *
by Neville Coleman
Softcover
Available from Sea Challengers  or click here for google search
Published by Neville Coleman's Underwater Geographic Pty Ltd
Publication date: July 2001
ISBN:
0947325255
read this review from the Sea Slug Forum's Dr. Bill Rudman

* updated version is titled Nudibranchs - Encyclopedia - Catalogue of Asia/Indo-Pacific Sea Slugs
by Neville Coleman
Hardcover, 416 pages
Available from Sea Challengers  or click here for google search
Published by Neville Coleman's Underwater Geographic Pty Ltd
Publication date: 2008
ISBN:
9780947325411
read this review from Underwater Australia


Coral Reef Animals of the Indo-Pacific - Animal Life from Africa to Hawaii Exclusive of the Vertebrates

by Terrence M. Gosliner, David W. Behrens, and Gary C. Williams
Softcover
Available from Sea Challengers  or click here for google search
Published by Sea Challengers
Publication date: 1996
ISBN: 0930118219
Comments: Includes 115 high-quality nudibranch photos w/identification (not including flatworms), with 1150 total photos of various reef invertebrates. Excellent introduction, photographs are all of excellent quality, with species identification, natural history, and distribution.
Common and Scientific Names of Aquatic Invertebrates from the United States and Canada: Mollusks
by D.D. Turgeon et. al.
536 pages, 2nd edition, with companion CD-ROM
Published by American Fisheries Society
Publication date: 1998
pages: 536
ISBN: 
1-888569-01-8 (paper); ISSN 0097-0638
Comments: A reference volume for malacologists; a long index of 6,200 scientific and common mollusk names with 50 color pictures. See the American Fisheries Society order page for this book

Indo-Pacific Coral Reef Field Guide
by Dr. Gerald R. Allen & Roger Steene
paperback, 378 pages
Published by Odyssey Publishing Company
Available from
Sea Challengers or click here for google search
Publication date: 1994, '95, '96
ISBN: 9810056877
Comments: limited nudibranch & sea slug section but a great overall reef identification guide, with 1800 color illustrations, from marine plants to fishes to birds, and seemingly everything in-between
.
Nudibranchs of the World
by Helmut Debelius & Rudie Kuiter
Hardcover, published 2007, 360 pages
ISBN 3939767069
available in USA from either
Sea Challengers (item #Item #221DK-R) or click for google search 
review available from Underwater Australasia, halfway down this page


Nudibranchs & Sea Snails - Indo-Pacific Field Guide
by Helmut Debeluis
Hardcover, published 1996, 321 pages
available in USA from either
Sea Challengers or click for google search 
Comments: excellent photographs by dozens of underwater photographers, very well researched and illustrated; a must-have book for those interested in nudibranchs.



Nudibranchs of Southern Africa : A Guide to Opisthobranch Molluscs of Southern Africa
by Terrence Gosliner
Paperback, 136 pages
List: $37.95 
Published by
Sea Challengers item #117G or click here for google search
Publication date: August 1987
ISBN: 0930118138

Pacific Coast Nudibranchs : A Guide to the Opisthobranchs Alaska to Baja California **
by David W. Behrens
2nd Edition
Paperback
List: $25.95
Published by
Sea Challengers item #2B 
Publication date: April 1992
ISBN: 0930118170
Comments: An excellent section on anatomy with plentiful color photos throughout; includes all known species of nudibranchs, with photos, from Alaska to Mexico; 217 color photos. The definitive publication about nudibranchs.


** updated version is titled:
Eastern Pacific Nudibranchs : A Guide to the Opisthobranchs Alaska to Baja California           
by David W. Behrens (Author), Alicia Hermosillo (Author) December 2005
available from the publisher, Sea Challengers or click for google search or 
title-specific Amazon.com search

See this book review from
Hans Bertsch, Research Associate, California Academy of Sciences

Pacific Coast Nudibranchs - Supplement 1 - Radula
by David W. Behrens
Softcover
List: $6.95
Published by Sea Challengers* item #2B - Supp
ISBN: ?

Sea Slugs of Western Australia
by Fred E. Wells and Clayton W. Bryce
Softcover, 184 pages
Available from
Sea Challengers  or click for google search
Published by: Wester Australian Museum Press
Publication date: 1993
ISBN: 0730955230
see this review by Wayne Ellis



Hawaiian Nudibranchs
by Hans Bertsch and Scott Johnson
Paperback  (out of print, hard to find), 112 pages
Pricing unavailable
Published by the Oriental Publishing Company, P.O. Box 22161, Honolulu, Hawaii 96822
Last known publication date: April 1981 (first edition)
ISBN: 0932596150



Children's books on sea slugs:

Down in the Sea : The Sea Slug
by Patricia Kite (children's book)
School & Library Binding, 24 pages
List: $14.95 
Published by Albert Whitman & Co
Publication date: April 1,1994
ISBN: 0807517178


What on Earth Is a Nudibranch? (What on Earth)

by Jenny Tesar (children's book)
Library Binding
List: $14.95
Published by Blackbirch Marketing
Publication date: April 1995
ISBN: 1567110991

Sea Challengers address:

Sea Challengers
5091 Debbie Court
Gig Harbor, WA 98335
USA

253.851.4179 Voice Mail
253.851.5677 Fax

Ask for a catalog from Sea Challengers through their online form  -- they have a wide selection of over 250 books for biologists, taxonomists, naturalists, students, and divers.

Sea Challengers is owned by Eastern Pacific Nudibranchs author Dave Behrens and his wife Diana.  Sea Challengers is my first choice for ordering books about nudibranchs or any other sea creatures. Their prices are competitive and they definitely have the best selection.

The Sea Challengers web site is at http://www.seachallengers.com.