The game is currently in Early Access, but you can download a demo from the CE Steam page, read more about the award win here, and see the current trailer here. I can also highly recommend PalicoPadge's playthrough series on YouTube.
Is this a brand new story? No - this is the same story that was serialised in The Phoenix Comic back in 2013 (issues 75-78), though I have re-edited a bit of the text, and re-written and re-drawn large parts of two pages (pages 17 and 22 in the book).
Is this a full-length album? The actual Secret of the Samurai story is 20 pages long, but the album also contains two other short stories I wrote and drew (both of which have been published before): The Sword of Truth (2004, 6 pages) and The Girdle of Polly Hipple (2005, 4 pages). The album is 36 pages in total.
What is the book's availability? I'm not sure on this yet - as I write the album is not currently on the BD Must site. The French language edition is listed on Amazon. A crowdfunder for the French edition was successfully funded back in May on Ulule. I do know the print-run is not large, and that it's smaller for the English language edition. I will update this when I know more.
What's the story about? Briefly it's a mystery about the hunt for a missing set of samurai armour, and it takes place before The Rainbow Orchid. For more details and notes you can see some of the blog posts I wrote at the time it was serialised in The Phoenix Comic (note - some of the info in these posts is no longer accurate!): The Secret of the Samurai FAQ, blog post for part 1, blog post for part 2, blog post for part 3, and blog post for part 4.
I'll end off on the other question I get all the time (and no, I'm not tired of it, it's lovely), Will there ever be a new Julius Chancer adventure? The answer to this is yes, as long as I don't get knocked down by a bus (or whatever the modern-day equivalent is ... an Amazon delivery robot?). I'm still very busy at the moment working on The Curious Expedition 2, but I haven't forgotten Julius Chancer - the new story is all-plotted, semi-scripted, and drawing started (extract below). I fully intend for this new story to happen ... I just don't want to promise when!