I found it! Mermaids of Japan, by Frances Haar, Kanabeshobo Company, Tokyo, 1954.
Not the book itself, but my photocopy of most of it. Including the title page. (Which I made at the time on the basis of, I wanted the info for book research, the library who had it definitely wasn’t going to remainder it, and even then I had a good clue this was a rare book I might never find again.)
Backing up a bit – I spent some time rummaging through my boxes of books so I could try to move everything related to the alternate-magical-history idea in one place. That way when I get more books for research, I can hopefully get them effectively, filling in the cracks of what I don’t yet know. So here are some more things I’ve read in the past leading up to this idea.
Yoshiwara: The Glittering World of the Japanese Courtesan, by Cecilia Segawa Seigle. Starts before Edo was a capital and goes through the start of Meiji. Not exactly a straight-line history book, it covers a lot of what people were doing in the rich sections of Edo, what costs it had in money and people, and gives a lot of insight into how the Ishin Shishi could do a lot of their plotting there without the Tokugawa Shogunate immediately catching on.
I’m going to put a side-note in here that this is one of several interesting history and history-related books printed by the University of Hawaii Press. Maybe it’s where they are physically, maybe it’s the fact that they can draw on more scholars than usual who read and speak Asian languages, but they have a lot of stuff on Asia, the Pacific, and the whole Ring of Fire area that you may not find elsewhere. If you want to look at history outside what’s taught in regular K-12 schools, look them up on their website or on Amazon. I’d bet you’ll find something you want to know more about.
The Mongol Invasions of Japan, 1274 and 1281, by Stephen Turnbull. Yes, it’s an Osprey Publishing book, meant to give wargamers background for their battles. It’s also a seriously researched book, with illustrations and maps – writing and wargaming have a lot in common in that those are very handy. It’s also pre-1618 (which is when the first story is going to be set) but there’s a reason for that. If I’m positing a parallel Earth where most of history went as we know it, but it now has magic, I need to know where the “break” point was so I can find all the possible ripples. Historically, there was a Great Comet in 1264 that sounds like a good bet for “and then things got very weird.” Note that comes before the Mongol invasions through Korea to Japan. (Though the Mongols had been invading Korea since the 1230s.) So I need to take it into account.
The Composite Bow, by Mike Loades. (Also Osprey.) Even with the advent of gunpowder, composite bows are a respectable weapon and still well-used in the time period I’m poking. They’re portable, powerful – and if you end up facing monsters that need specific banes to take down, it may be easier to do that in an arrowhead than in musket shot. (If it’s a really big monster, I’m seeing possibilities of mixed units, one giving cover fire while the other reloads, both of them buying time ‘til you can get a hwacha aimed the right direction.) It’s also a modern hobby I can give my poor isekai’d guy – though he’s going to have to learn a lot more….
The Conquest of Ainu Lands: Ecology and Culture in Japanese Expansion 1590-1800, by Brett L. Walker. He also wrote The Lost Wolves of Japan.
Another small digression here. I ran across Noel Perrin’s Giving Up the Gun many years back, which was a paean to how Tokugawa Japan voluntarily giving up firearms made technology go back to a pace humans could live with. And said to myself, “That… doesn’t sound right.” In my experience, at least, people do not voluntarily give up the capacity for violence. That way leads to mass graves and all kinds of nastiness. And yet in some circles (mostly of authoritarian mindset) the Tokugawa Era is held up as a model of ecological responsibility and a population kept with its natural limits, all because only the people in charge had all the force available.
Except if you study the actual history that is not what happened. This book, Conrad Totman’s various works, and others – especially those works covering trade between Japan and Asia – show that Tokugawa Japan was constantly pushing up against ecological limits and finding new ways to exploit more of them. They lost a lot of population due to the Onin Wars, and then more to the Warring States era before the Tokugawa closed the country, they imported a lot of agricultural innovations (and new food crops!) through trade with Europeans and Asia, and they took over the northern islands and the Ryukyu Islands, so their population growth mostly kept within the limits of food production. Even so it was a bumpy ride sometimes, and straining at the limits by the time of the Meiji Revolution.
Get sources for your writing. Get lots of sources. Never depend on what “everyone knows”. Check it out!