I've had it for two weeks now and I'm about half way through reading it. So time to give mt thoughts on it. Many people may be familiar with the previous Bitmap product- The CRPG Book. This is the same sort of layout and style, but focusing specifically on Japanese Role Playing Games. The Bitmap website has a good overall description with sample page photos.

Physical quality is outstanding. Full 10/10 as far as I'm concerned. Heavy weight paper, hard cover, stitched binding. It weighs a shit load*, is 650 pages and is not something you're going to be able to casually carry around in your backpack for light reading over lunch. Though your purchase does come with the right to download a PDF file for portable reading.

*The "shit load" is like an official unit of measurement in Australia. You will have to covert to your local imperial or metric units yourself.

The meat of the book covers the history of JRPG's. It's meant quite literally too. Japanese RPG's in that they are from Japan. Some Western games that copy Japanese styles are covered in the final chapter, more on that later.
Chapters cover:
-Early Japanese PC games like PC88 etc. More of historical interest than anything for me, as I'm unlikely to look for any of these to play.
-Larger series. Dragon Quest, Final Fantasy, Shin Megami Tensei and spin-offs. TalesOf series.
-Other Franchises. The largest chapter by far. Concentrates on traditional turn based JRPG's that don;t come under the already covered big four.
-Action RPG's. Like the previous chapter but for action RPG's of all types- everything from Secret of Mana to Dark Souls.
-Strategy RPG's. Fire Emblem, Shining Force, Front Mission, Final Fantasy Tactics etc.
-First Person Dungeon Crawlers. Japanese Wizardry style games like Mary Skelter, Etrian Odyssey etc.
-Three smaller chapters covering Japanese Roguelikes, Monster Catcher games like Pokemon and a Miscellaneous chapter for things that don't fit- like MMO's and a few western games that ape Japanese styles like Fell Seal or Anachronox.
Even only halfway through I already have my own list of games that I want to track down- like Front Mission, Parasite Eve and especially a mega shitload of DS/3DS games that I knew nothing about (since I recently got a New 2DSXL).
I'd rate the bulk of the books content at 9/10. Only a few little nitpicks hold it back from perfection to me. The main one being I'd have loves some form of instant method of just looking to see if each game has any English localisation. It is covered in the text descriptions sometimes, but not always- it's inconsistent. I'd have liked one more entry under each title where the dev and plaforms are mentioned that says Localized: Yes, No or Fan (for unofficial fan mods). It's just a bit annoying to read about a game and think "I want to play that, is it in English?" and have to research myself.

Who's it for and is it worth it? It costs less than a single AAA video game. The cost is worth it for the quality, no doubt in my mind. Depending where you live though, postage could almost double the price- but they do use DHL (well to Australia they did anyway) and it's fast delivery.
Whether the book is worth it to you though depends. If you hate JRPG's with a passion, then no. Obviously. If you have a passing interest in them and also just video game history, then maybe. If you love JRPG's, it a no-brainer, you should love it. If you thought The CRPG Book was awesome, no reason why you won't think the same for this.
I really hope Bitmap now continues the style for other genres: Strategy Games would be my preferred next subject, followed by Simulators and First/Third Person Shooters.

Oh, so what's their definition of "JRPG"? It seems they include blobbers like Etrian Odyssey so any RPG made in Japan counts?

Don't know, but it can't be a legit one since the OP claims they are calling Dark Souls a RPG and/or JRPG. But it's not. That's a straight-up action game.

Probably akin to walking simulators and other games of being 'immersive' with 'in depth story telling' if you really look around, like Gone Home.

If it's an RPG and made in Japan it's a JRPG.

Bioware moves their studio to Tokyo, but changes none of their staff. Would that make the next Dragon Age a JRPG?

If Diablo is an RPG? Then so is Dark Souls. If the Witcher 3 is an RPG then so is Dark Souls. Everything in Dark Souls is effected by your stats, except your aim and dodge timing- and even that is still effected by your character build now that I think about it. Everything about the way you build the character effects the way you play. How is that not an RPG? All abilities range from 0-99 and effect the things you do and the effectiveness of your equipment. Pure action games, stats don't effect anything. Sekiro is borderline RPG maybe, but Dark /souls there is no doubt- finest action RPG ever made.

I think the argument is over diehard point-point definitions of genres that have become pretty diverse and deep within themselves. The modern crop of 'rpgs' tend to be more action-focused with little care for stat-based mechanics and dice rolls in general beyond 'yo you hit for 11, 14 then a crit!'

We've seen similar arguments breakout in FPS circles, RTS circles and driving stuff

I'd have to say no. You might look at the company's origins to determine that, but combination of belief values and ingrained history and lore will have a distinctive difference in a game's creation. You can't just replace sandwiches with rice balls, swords with katanas and 'enemies' with 'ninjas' and get the same thing across.

I suppose it's more psychological in nature. You can become a cathlic (or whatever) in a day, but the mindframe, habits actions and how you approach things hasn't set in. For Bioware to make JRPG's after moving to Tokyo would probably have to appease to the local audiance first with products, then make a game with the same quality/clarity/mentality.

Now if EA bought a Japan company renamed it to Bioware Japan, and they made a game, sure it probably would be, though how buisness would work across those lines i couldn't tell you... Though the expectations of a Bioware game would be quite skewed...

Fried seaweed and green tea anyone? Hmmm... Let's break it down to it's base components. RPG, Role Playing Game. The key word being Role. So what is a role? Is a fighter a role? A healer? Or is it the politics of the character. So...

Healer: One who heals, improving HP of their party
Healer politics/character: A priest, cleric, or other person who would donate to the church, do good acts, try to save the town and people at the expense of his own life.
Healer's backstory: He was a orphan and grew up poor, being taken in by the church. He learned magic and did healing as a service to those less fortunate than him.

With Pen and Paper games, RPG usually is synonymous with playing a single character. Unlike large scale war games where you do squads in WW2 fights in turn combats, you instead play a single character. But how far you can play them can be quite loose. Very vague, you play a character who is in the party and hangs around healing (like a bot) or someone trying to save a bugbear's life because his attack was based on something else causing the act and not because it was evil... Both are valid, though you still only play one character.

In video games, RPG games usually Final Fantasy is one of the better well known ones, but it's a linear story with a group (1-8 something characters) who interact and usually do battles.

Diablo/Diablo 2. You play someone who is given quests and has ONLY combat ability.
Dark souls... you die a lot, but you also have high combat ability...

Diablo/Dark souls is more action/adventure, less RPG. I guess the better question is, do you play to kill the things? Or do you play to experience the story? If you play to kill the things then it's not really an RPG but has some familiar elements. If you play to experience the story and there's killing the things, then it leans more RPG...

Takes me back to Castlevania Symphony of the night. I play to kill the things, but it happens to have a story... I wouldn't call it an RPG.

Meh, this is mostly ramblings...