"How Al-Anon Works" is the "big book" of Al-Anon.
I think it's worth reading the AA Big Book just for the information and insight it gives into alcoholism and recovery. The only danger is that you can be tempted to "helpfully" point out passages to the alcoholic! If you can resist that temptation, it's extremely helpful and informative.
One other caveat--even though the same twelve Steps are used, the way they are interpreted and worked in Al-Anon is slightly different. Al-Anons tend to have slightly different issues from the ones faced by alcoholics in recovery. But lots of us who qualify for membership in both programs find benefit to the Steps from slightly different perspectives.