Phintias is a redgaurd bookseller and owner of the First Edition book shop in the Imperial City. It is the largest book store in Cryrodill and as such he is not afraid of competition and will gladly recommend other book stores.
During the path of dawn quest you’ll need to seek out an expert on books and are directed to Phintias by Tar-Meena to obtain a copy of the ultra-rare volume 3 Mythic Dawn Commentaries. He happens to have a copy but it was ordered and purchased by another customer who is supposed to come pick it up. After some persuasion he’ll hand over the book saying its better off in the hands of a ‘scholar’ than a ‘collector’.
During the order of the virtuous blood quest Roland Jenseric directs you to Phintias to obtain information on the where abouts of Seridur since he frequently visits the place. Phintias says Seridur regularly seems to travel and mentioned something about Memorial Cave.
For the tears of the savior quest the book Knightfall will need to be purchased from the First Edition as well as the book the Armorer's Challenge for amour training from a master.