The Factory R-21 Owners Reference Manual is the Greatest. There are 3 Routine Maintenance Manuals that the typical DIY owner/mechanic also needs: (1st), A "Parts Manual" is required for direct access to part numbers and part breakdown diagrams. (2nd), A "General Maintenance Manual", to get a general description of the numerous systems and important servicing info with trouble shooting for each. (3rd), A "Wiring Diagram Manual" will have the factory original wiring diagrams and associated trouble shooting guides. There are several other Maintenance Manuals that cover overhaul & heavy maintenance (Engine, Transmission, etc). Does anyone know where or how to find any of the basic routine Maintenance Manuals? I've checked with Volvo Penta, West Marine, etc. Who Knows? BB+