- Arthur William Whatmore, Insulæ Britannicæ: The British Isles Their Geography, History, and Antiquities Down to the Close of the Roman Period (1913). Gathers almost every classical reference to Britain and Ireland. Also contains fun statements like "The story of the Sirens is a play upon the Gaelic word 'seirean,' applicable to the promontory of Lleyn, Carnarvonshire." Locates Atlantis in Ireland. Proof that if you learn too many dead languages, they will gang up together in your head, take over your neurons, and make you expostulate wacky things. Only you'll do this in Greek, Hebrew, Welsh and Latin, so no one will really notice.
- Howard Rollin Patch, The Other World According to Descriptions in Medieval Literature (1950). Finds the Rig Veda alive and well in Irish mythology. Thinks the Welsh, the Scots and the Irish are all just the Celts, while the Icelanders are the Germans. A pretty good comparative overview, though, of lands of the dead and other kinds of otherworlds in various medieval myths.
Everyone: what's your favorite antiquated and little known book, with just that right mix of the time bound and the eternally useful?