Yen Press' The Time I Got Reincarnated as a Slime: The Ways of the Monster Nation Vol. 1 is a manga spinoff of the hugely popular and successful light novel series The Time I Got Reincarnated as a Slime. The mothership series spawned a globally popular anime series, a manga series adaptation, and already another manga spinoff. The Way of the Monster Nation is yet another manga spinoff.
To recap, the main series was an entry in the isekai genre where a person from our world ends up in a fantasy medieval world that follows the rules of Dungeons & Dragons and video game role-playing games. The main character often knows the rules of the world because they're a gamer, which gives them a massive God Mode level advantage over everyone else in the world, who are unaware they exist in a D&D world. They get the chance to be an epic hero in this world in a way they never could in our world.
The Time I Got Reincarnated as a Slime by author FUSE and character designer-artist Mitz Vah was about a thirtysomething middle manager who dies and gets reincarnated as a slime in such a fantasy RPG world. He becomes nearly all-powerful as he levels up and befriends all manner of people from the various species you meet in this kind of world: orcs, elves, demons, demigods, rabbit people, lizard people, you name it. Instead of being a hot-headed fighter, Rimiru, per his new name, befriends and elevates the different species. He's a middle-aged guy who prefers to avoid conflict, so he uses his managerial skills to make everyone get along instead of fight and kill each other. He ends up building a thriving kingdom called Tempest, where travelers and citizens of all species can live and trade in peace.
The Way of the Monster Nation is a spinoff for the most hardcore fan only. It's a slice-of-life story entirely made up of fanservice. You have to understand that "fanservice" in Japanese geek circles often means female characters in sexy poses purely to titillate male readers, and manga creator Sho Okagiri is catering to that market. The protagonist is a new character created especially for this series, a rabbit girl named Framea, the daughter of the rabbit people's leader. The latter comes to Tempest at the request of Rimiru. He tasks her with writing about the best and greatest wonders the world has to offer for a guidebook he plans to distribute to the people of the world. This includes food, monsters, perils, locations good and bad, and everything in between. Framea interacts with the supporting characters from the main series before she sets out on her own adventure with a team of adventurers while Rimiru watches over her from afar in a way that's totally non-stalkery, I think.
The problem with this story is Framea is not the most compelling character. She's a cipher beyond being every nerd's cheerful fantasy girlfriend who's endlessly chipper, earnest and eager to please. The team she ends up with is also the usually cheerful and earnest bunch of adventurers you find in every manga series. The most interesting and charismatic character is still Rimiru, who's a supporting character and more of an extended cameo, even though his presence is felt throughout. If you didn't know about the original mothership series, you would be at a loss for why anything in the story matters or who everyone is. This spinoff is that it's for hardcore fans only. Even if you are a hardcore fan, you might be left hankering more for the adventures of Rimiru and his helpers as they fight and politick their way through the kingdom rather than Framea's bland and generic tourist's journey and mild D&D adventures.