Erler's Germany series consists of 18 volumes. He also published a couple Germany-related volumes that aren't listed as part of the 18-volume series, (ex. The Stamps of the NSDAP [Nazi Party] and its Sub-Organizations, 1995). Some of the older ones are very difficult to find, but the more common ones can still be found new.
Vol. I Deutsches Reich, 4th edition, 1998.
Vol. II German Colonies & Overseas Steamship Lines, 5th edition, 1998.
Vol. III German States: Anhalt to Kurhessen, 2nd edition, 1991.
Vol. IV German States: Lippe to Wurttemberg, 2nd edition, 1992.
Vol. V Danzig, Memel, Oberschlesien, 1995.
Vol. VI Saargebiet, 1981.
Vol. VII Court Fee Stamps after 1945 (1946-1973), 1981.
Vol. VIII German Occupation Issues 1914-1919 / 1938-1945 & Allied Occupation Issues 1947-1951, 2nd edition, 1996.
Vol. IX Fee Stamps of Bavaria Issue of 1957, 1982.
Vol. X General Municipal Fee Stamps, 2nd edition, 2000.
Vol. XI Berlin, 1st edition, 1990.
Vol. XII SBZ/DDR, 1993.
Vol. XIII Municipal Revenue Stamps of the Saargebiet, 1992.
Vol. XIV Social Security Stamps: LVA Provincial Issues, 1994
Vol. XV Social Security Stamps: Issues from 1.1.1923 On, 1994.
Vol. XVI Social Security Stamps: Saargebeit.
Vol. XVII Freight Stamps, Newspaper Parcel Stamps, Seat Reservation Stamps, Tax Substitute Stamps, 1999.
Vol. XVIII Municipal Fee Stamps, 1999.
There are a few dealers that carry them, though you'll likely find only the newer, more common volumes. Try
Werner Zielniewicz,
Phil Bansner, or
Eric Jackson.