I had heard rumours that there might be another book after Celestial Realms as well, before Edge takes over.

Slated for August, but I hope that they surprise us and get it out sooner than that.

I don't care what it is, why it was introduced: I like it.
Overall, I like this system. I like this fifth edition. It's the one that made me start game-mastering. It's important to me.
I felt like saying this after seeing so many negative comments.

I don't care what it is, why it was introduced: I like it.
Overall, I like this system. I like this fifth edition. It's the one that made me start game-mastering. It's important to me.
I felt like saying this after seeing so many negative comments.

I don't care what it is, why it was introduced: I like it.
Overall, I like this system. I like this fifth edition. It's the one that made me start game-mastering. It's important to me.
I felt like saying this after seeing so many negative comments.

I don't care what it is, why it was introduced: I like it.
Overall, I like this system. I like this fifth edition. It's the one that made me start game-mastering. It's important to me.
I felt like saying this after seeing so many negative comments.

I don't care what it is, why it was introduced: I like it.
Overall, I like this system. I like this fifth edition. It's the one that made me start game-mastering. It's important to me.
I felt like saying this after seeing so many negative comments.

@Avatar111is right, it's one the problem with this edition of L5R, rule-wise: there's informations everywhere, about everything, in margins, really, everywhere. A lot of time you don't know where to search. It can really be an hindrance when playing.
That and calculing strife in battle for opposing PNJ can really drag the game unnecessarily.
A possible fix would be a "companion" or "player's guide" type of book, ordering things a little more.
Anyways, seems those books are finally gonna be released. It's good, I was begining to lose hope.

What comes to mind when I read that adventure. 

I'd be counting on that being the Lion Clan book, presuming we get there. Seems like it would fit there best.

Yes a new sourcebook!
The map of the realms does look amazing, hopefully the back of the map of Phoenix territory has at the other side the map of the realms.
A good and logical choice to put the Centipede clan together with the Phoenix.
With this book, there seems to be no pattern which sourcebook they are releasing.

At the least I was hoping for a monk/shugenja book and something for more in depth mass battle/bushi focus.  And a Monster Manuel/encounter book would have been very handy to see.

Well, frankly that makes things a lot easier - since there's no shujenga or courtier, you've got no 'squishy' characters who only get a wakizashi and have to break out the pleasepleasepleaseloanmesomearmour courtesy checks whenever a battle is in the offing.
 
 
The hida is tough and nearly immune to critical strikes, whilst the Ronin and Shiba are both able to last an extended fight without becoming compromised (assuming - since you described them as a Bushi - the Ronin's signature skill is Martial Arts [Melee]?). The Shinjo arguably has the weakest ability but it's most easily applicable outside a fight.
Yes, with 4 bushi, you will need to amp up martial checks to seriously threaten them. If they're rank 1, then - without rolling dice - it's clear that a group of 4 Desperate Bandits (4 x 1 threat) will struggle to last a single round. As noted, I would suggest that if you want to make them at least sweat a little, look for 'actually had to concentrate' fights being of half again to double their collective rank (6-8 bandits, operating in groups of 2-3 for speed and simplicity*, or 5 with an Experienced Bandit leader to use ....Get Them, You Fools!)
 
 
 
I would probably advise that if you want to really challenge them, throw in some intrigues. With four Bushi, they're going to find an equivalent political scene much more difficult - and a seasoned courtier with the Trickster template (political threat 7) can probably out-diplomat the lot of them combined without breaking a sweat.
 
Also, setting the campaign - at least temporarily - in and around court means the PCs will be dealing with social protocols around weapons and armour, which means Ashigaru armour (for the Shiba and Shinjo) is a no-no, as are double-handed weapons (for the Hida), wearing lacquered armour may be broadly permissible but makes some big - and deeply negative - statements about your opinions of the host, and no, you can't bring your ****ing horse into the inner palace....
As a result, they should be largely forced to operate with Daisho and formal robes unless they have a VERY good reason to gear up, so in a putative skirmish where a bunch of hired ruffians or mercenary shinobi try to scrag them in the gardens or a tea-house somewhere, you can force them to fight smart because they can't rely on the bushi wargear they're used to having.
 
 
* Most minions who aren't in groups of at least 3 will probably not be able to hurt Sugi and will struggle to hurt the Ashigaru armour of the others.

Sorry, work was demanding.
I have a Shiba bushi, a Shinjo outrider, a Hida bushi (Hida Sugi from the box, the player instantly fell in love with her) and a rônin bushi.
I had also a Bayushi courtier but he missed three consecutive sessions, so I'm afraid I'm gonna have to cut him out.

Regarding balancing opposition, I think that despite the corebook general guideline, you need to have a solid grasp of the system and understand your party's capabilities.
It also depends on the amount of players you have. There are so many factors, like, for example, it is possible an otherwordly nemesis would be a push over for your group because some have sacred weapons, or techniques like Binding the shadows etc.
There are no "right" answer here... The solution might be adding more minions, or crazier terrain, or stronger adversaries with encounter changing techniques, or more counters to your players strong shenanigans.
It is case by case.

D&D 5e did it smartly. Released a corebook that was very polished and easy to learn, with a system that is easy to play, kept the crunch addition to a minimum in the sourcebook. The rest is setting stuff or "open source material" for people who likes crunch.
And, if you do not want to play a character who reads the book, you just play a non spellcaster.
L5R doesn't have that accessibility, cleanliness and polish. It is actually kind of unwieldy. Plus, it requires one **** of a good GM to run (even more so if the players are not familiar with the setting).
If you have a heavy setting game like that, you need a more streamlined system like blades in the dark, much more elegant and focuses on creating narratives.

Only 4 more days until the end of January, wonder FFG will pay their shippers express delivery again? Also I hope this won't be the last L5R Book, because I really like this interpretation of the system. 😣