No doubt about it, Brian has some good prices on the manuals that he has. Plus great service!
Another good source with reasonable prices (IMO) is MILMAN at ebay. Check out his store at: stores.ebay.com/Manuals-and-Stuff
As an example, he has SB 11-131 (vehicular radio sets/installation) for the VRC-12 family, etc. for $25. This is one that we can't download from LOGSA. Plus his shipping charges are reasonable, unlike some ebay sellers.
Yes, C'dub the military (contrary to some peoples opinion) does have its strengths - having a systematic approach to things is one of them. Data plates will show TMs and other reference numbers, just as TMs will show related TMs, etc.
Speaking of those manuals for the M923 - they are all available in .pdf for free download at:
www.logsa.army.mil. enter 9-2320-272 in the manual number (without the TM) and you will get the list for the TMs, MWO, LO, etc. While I like the hard copies, the price on the ETMs is great if they are available for public release.
What other county would give us all this good information for free?
USA! USA! USA!