Callahan's is a 11-book series with 9 primary works first released in 1977 with contributions by Spider Robinson.
1977 • 42 Readers • 224 pages • 3.8
1992 • 3 Readers • 352 pages • 3
1996 • 3 Readers • 214 pages • 3.3
2000 • 1 Reader • 335 pages • 2
2003 • 2 Readers • 286 pages • 4
1994 • 1 Reader
1994 • 1 Reader • 5