A few recommendations:
A- Use a tape recorder instead of trying to take notes, you can cover a lot more ground that way. A video tape is much better than just voice tape. Look at all the great stuff on YouTube that covers anything from a single topic for a couple of minutes up to hour long discussions. It may be easier to edit some stuff and leave it as video recording than trying to convert to words printed on paper. Or to do some printed and some video, overlapping or entirely different topics.
B- Get as many old photos as you can and try to get him to at least ID them when/where/what/why/how. You cannot do that yourself, and looking through that will trigger memories and stories.
C- Ask about some of the other cartridge or gun people he interacted with. Tell us about them, their lives, interests, quirks and funny stories.
D- A “book” is no longer always a printed item. An “E-book” is a whole lot easier to distribute and cheap to produce and easy to edit/update. If you decide on printed format, talk to Mel Carpenter who has done an outstanding job on his Gyrojet book and HWS volume 3, a perfectionist on technical details.
E- However, even an amateurish account with flaws in execution, spelling and typo errors and some possible factual errors which reaches the “published” stage in whatever format will be a far better contribution than the “perfect book” which never gets completed and ends up as a jumbled pile of notes and incomplete sections which is eventually thrown in the trash. Aim for a level of detail, publishing perfection, and completion date that YOU are able to achieve and go for it.
F- Machinery is a good topic that seldom is discussed at all.
G- Get started immediately! Tempus fugit and all that.
A very interesting and worthy project.