As a starting point for ops manuals, two old ICAO documents and a related GAMA document for some history. As I've not been in that area for many years, I've no doubt that later versions exist and should be referenced for current requirements.
Also, individual Regulators will have a variety of guidance material - eg
CAAP 215-1(2) for Australian operators.
(a) Doc 9516-AN/930 (first ed at 1991).
Guidance on the Preparation of a Pilot's Operating Handbook for Light Aeroplanes
(b) GAMA Spec No. 1 Specification (initial issue Feb 1975 and revised Sep 1984)
Specification for Pilot's Operating Handbook
(c) Doc 9376-AN/914 ((first ed 1990)
Preparation of an Operations Manual
The ICAO index can be found on the
ICAO website.