Where can I find (preferably free) online resources, such as How-To-Write or styleguide articles and blogs, about writing a litRPG book?
LitRPG, or Literary Role Playing Game, books combine elements that you would find in RPGs with science-fiction or fantasy. This means that the reader is made aware of what the character in the book sees, such as tables with the different stats or messages by the system that show who attacked what with how much damage.
An important aspect is also that these books often describe the player and the character alike and therefore switch between different points of view to explore the role of RPGs in our society, while at the same time depicting the fantasy or sci-fi elements that the in-game character experiences.
This puts the writer in a situation where they have to think about designing the most important aspects of a game and especially how these mundane things are presented to the reader of the book in an interesting way. The combination of a critical real-person plot with a less-serious fantasy-character-plot is another aspect that the author has to keep in mind.