Primarily B'jaern's mother's tribal pendant is his most prized possession and his only memento of her.
Hopefully they have a God' Quiver armor set available in the near future, because I did see a Wood Wailers coat on XIVDB. Other items might include his toolbelt, bow, and three specific books which he alternates writing entries in.
Raimdelle's Codex / Bestiary-similar to Hunting Log
The Portly Moogle's Lexicon: A guide to moogle lore and behavior.
Chocobo Care & You(Breeder's Guide)