M is for Manual
There are so many manuals!
Central BC Railway and Forestry Museum holds hundreds of operating and maintenance manuals for many industrial machines, locomotives, and pieces of rolling stock.
In these documents an operator, mechanic, or researcher could find many different categories of information such as electrical diagrams, fuel and lubrication systems, cooling water and air systems, lubrication schedule, tests-circuits, parts catalogue, and this is only to name a few examples!