This question is unlikely to get a strong answer in this subreddit, as it is extremely niche. As a weird nerd who enjoys this kind of thing, I am going to err on the side of giving you at least a little information. This is not my flair subject, but I know enough to drop a couple of tips.

The only books I know of on this topic are in Japanese. A Brief History of Japanese Hotels, by Minoru Muraoka, was published in 1981 and contains information on then-recent hotels. Showa Dignity: The Secrets of Classic Hotels by Yumi Yamaguchi is another good resource, although the hotels were mostly from the early Showa era, and are still running today, so there have definitely been renovations, but it's still got really nice visuals.

And while this is not an academic source, this listing focuses on "Showa retro" hotels you can visit. There is still a lot of nostalgia in Japan for late-era Showa, which a lot of people remember quite well, so you can find some pictures there of hotel rooms that are specifically meant to evoke the period you're talking about.

If anyone does have a better answer, please do give it, I'd like to know more too.