The owners manual for these cars is quite complete and will provide you with a wealth of information.
There are two parts/pictures manuals available. The small one, the same size as the owners manual, will show the complete parts breakdown for the car, but with no individual parts numbers. You can usually find these on e-bay on a regular basis. Just watch to make sure you get one that is right for your car, I would also stay away from the reprints, as they usually show more updated information.
There is also a large parts book with all the parts number for the car but without the engine information. These also show up from time to time on e-bay but are generally expensive (150-200 range). There is a reprint available from Benz but I think it is for the later cars.
There are also other repair manuals, from Haynes (so-so), Kenneth Ball (decent) and Glenn's (also decent) which you can find on e-bay.
The other repair manual is referred to as the BBB (big blue book) and covers a range of vehicles from Benz. The one of interest covers cars from 1959 to 1967. It is written for a Benz shop, so it presumes a reasonable amount of knowledge about basic operations and mechanics.
There is also a technical manual available. This is a book that lists the various torques, clearances, capacities, settings, etc. for the car. If this is of interest, just make sure you get one that includes your year.
There are a lot of other general reference kinds of books, about restoration, racing, history, etc. But they will not have detailed mechanical info.
Jonny B
1967 250SL Auto