Sacrosanct
Legend
Use Google, man!I think the author note at the beginning is a good idea - maybe consider sidebarring it since people seem to notice sidebars more than preface or foreword text.
I'm not sure I'm keen on a monster by monster flag, at least not without context being identified. Maybe have a table of the monsters that have the flag and a keyword or two about why the monster will be flagged later in the book.
I do like the cultural source of the folklore under the creature's name, but I also think it would be nice to include a reference to whatever textual sources you're drawing from - like a citation of the books consulted at the end of the monster's description. Maybe a "For further reading" header. It would be a nice addition for people who see the glashtyn and say "Manx folklore?!? Where can I learn more?"
I jest. Well, partly...

In all seriousness, and I do mention this in the introduction, that there is so much overlap between cultures of basically the same creature, just with a different name or very slightly different aspect, that it was decided to go with a version that tries to encompass all of them into one. I do also mention additional sources in the introduction as well, citing MacCulloch, Yeats, and Briggs in regards to fairy lore, and Geoffrey of Monmouth’s Historia Regum Britanniae and Thomas Malory’s Le Morte d’Arthur for the Arthurian lore (as well as side mentions of the Volgate Cycle and others).