OK, so you got an old "What the heck is it?" or "What did this do?" item.
Well I have run into amore than a few of those over the years B.I. (Before Internet).
I find that at the hamfairs, estate sales etc, one can often find old Fair Radio (Yes, they are still in existence!) Catalogs.
Having the pack rat mentality, I have stashed away a number of their old catalogs/flyers. They have assisted in finding control boxes I didn't know the part number of, cables, and missing assemblies, etc.
Also of great help, and some really cool projects are the Ham Radio News type publications from GE, RCA, CBS and others. If you are into servicing old TV sets, the Sylvania News publications are killer. They even have listings for the new tube types for the Sylvania Model 123 series tube testers!
The GE and RCA ones really catered to the Ham crowd, with detailed schematics, parts lists, etc. for everything from power supplies, VFO's, monobanders, to linears.
Of course, the companies touted their own tubes, and often the newest ones they just made; but great info.
Another one was the CDE newsletters from Cornell Doublier called the CDE Capacitor. Mass good info- generally more than you want to know-, about all types of capacitors. Some engineering level stuff too!
OTOH, you can seldom go wrong by obtaining a few scattered years of the ARRL Handbooks. Typically, one about every 10 year period up to about 1970 for the old radio repairers. Some of it is a re-hash of older projects, but I found about every 10 years was a good selection of different projects, and the tube info in the back, esp. the transmitting types is really handy.
Editors and Engineers also made a Handbook, and it , IMHO, had a lot of better info than the ARRL handbooks, and more complex projects. I collected issues of both. Some of the E&E info was presented differently, and for me, easier to understand.
Of course, the old ARRL "Hints & Kinks" books were great. I have not seen any new ones for years, but I collected every different issue of them that I could find. Lots of forgotten information in them, and the earlier, the more useful tips were. (One issue even showed how to cut holes in, and drill glass!).