The Guestbook is an appendix to The Wandering Inn, updated one entry at a time and loosely following the chronology of Erin Solstice's story.
Description
The Guestbook is supposedly comprised of small "diary entries" written by innkeeper Erin Solstice about the various guests staying at the Inn. Later on, the Guestbook is apparently "hijacked" by Ryoka Griffin to write down some of her most notable deliveries, then by Selys to write down descriptions of adventurers she has had dealings with, and later stills by Relc Grasstongue for classifying criminals; both see the Guestbook taken back from them by an angry Erin.
In truth, the "guests" are named after (and their behavior is determined by) real-life readers of the book who supported Pirateaba on Patreon.
Continuity and Canonicity
It is at first glance unlikely that the Guestbook can truly be considered canon to the main novel, first and foremost because of the sheer amount of guests that would have somehow gone unmentioned in the main story (even some who are apparently supposed to have stayed at the Inn in the very beginning, before Erin had actual guests aside from Relc Grasstongue and Klbkchzeim).
Moreover, several of the Guests behave in ways that clearly indicate they come from Earth (such as "Dario", who "paid in dollars"), the existence of whom would negate the specialness of Erin finally meeting another traveller from Earth in the person of Ryoka Griffin.
That being said, the Guestbook does make an effort to follow the novel's continuity to an extent; for instance, "Caby" is the first one to visit the Inn after it was rebuilt closer to Liscor. Indeed, past the earliest chapters, the entires get more realistic, with Relc's criminals or Selys's Adventurers being arguably fully plausible as characters existing in the Innworld. The best way to look at the Guestbook is probably as being of a "lower-tier canon", such that information from it may be considered canon to the world of the novel as long as neither the main story nor Piratabea himself contradict it, implicitly or otherwise.