This article is from the Your New Dog FAQ, by Cindy Tittle Moore with numerous contributions by others.
There is very little material out there to help people who have
adopted older, grown, "second-hand" dogs. Some shelters may have
handouts for their clients. Carol Lea Benjamin has written _Second
Hand Dogs_, which is the only book published to treat the topic
extensively (and even then it is a relatively small book). Other books
that are of use are: Job Michael Evans' _People, Pooches, and
Problems_, which will help you if you have some behavorial problems
with your new dog. Another of his books, _Evans' Guide to
Housetraining Dogs_ contains some sections on how to housetrain grown
dogs. There are undoubtedly bits and pieces elsewhere in other books.