'Die Ahrtalbahnen' by Klaus Kemp is an excellent reference. This is an Eisenbahn Kurier book. The ISBN number is 3-88255-542-4.

The Eisenbahn Journal Sonderausgabe II/90 'Die Eifelbahn Koln-Trier und ihre Nebenstrecken' is also excellent. In this case the ISBN number is 0720-051X.

I just remembered that the magazine 'Bahn & Modell' issue number 9/89 contains a really good article on the Ahrtalbahn, 'Eine Reise durch das Ahrtal- Impressionen vom Vorbild furs Modell (2)'. Really good photographs, some drawings and good clear photographs. I have this issue but never found part (1). I presume that was in Bahn & Modell 8/89 (which I don't have obviously!).

Searching the web is bound to bring up other information as well.

Newcastle-Upon-Tyne and Netherlands