There's lots of times when knowing a little about what you're doing is helpful.

There's also some times when making a good fake at knowing what you're doing is of some use.

There also are some times when knowing more than you should can be sort of hazardous.

A decent manual can be a lot of help with the first two, and once in a while even with the third.

If your guy has been in the business for long, he may have the 1943 version of a lot of manuals, but for small engines they haven't changed enough to make a lot of difference so he's probably okay with it. For ordering parts, the "Illustrated Parts List" is generally more use than the manual, if there are separate ones you can get, but it's the one who needs the part who has to figure out which to look for.

If you just want to get what's available while something is new enough to make it easy, you should look for a "User's Manual," a "Repair/Maintenance Manual," and an "Illustrated Parts List" and make sure you get all three if they're available for the thing in question. If they want you to pay for a Repair Manual, it's generally okay to consider whether you have a "guy" who can take care of stuff that would be there, and "defer" that one if service is available. Those are the three things you're most likely to find, and there are good reasons for having all three when a manufacturer has offered them all.

Otherwise, just get what you can. If they're not all three available, enough part number info for anything you're likely to need will probably have been consolidated into one or both of the others, or for simple things it may all be in one booklet.

John