St. Dragon and the George

The Dragon Knight is a series of fantasy novels begun in 1976 by American writer Gordon R. Dickson. The first book, based on the short story "St. Dragon and the George", was loosely adapted in the 1982 animated movie The Flight of Dragons by Rankin/Bass. The title here refers in part to an in-universe nomenclature, wherein the story's dragons use the name "george" as a synonym or substitute of "human", after 'St. George the Dragon-Slayer', and in part a reference to the latter.

Baron Sir James Eckert de Bois de Malencontri et Riveroak, a.k.a. "the Dragon Knight" or sometimes "Sir Dragon".

The protagonist of the series, a minor academe originating in the 20th century, who to save his fiancée, Angela "Angie" Farell, follows her by astral projection to a parallel universe resembling medieval England, where his psyche is lodged in the body of a dragon named Gorbash. On consultation with the mystic Carolinus (see below), he is sent to rescue Angie from the immaterial evil known as the 'Dark Powers', and is mistitled a Baron by Carolinus (presumably to give him legal freedom of movement). At the end of The Dragon and the George, Jim regains his human form and obtains the land and titles of the deposed antagonist Sir Hugh, who re-appears later in the series.

In The Dragon Knight, Jim rules the estate Malencontri, with Angie as his wife; but finds himself capable of becoming a dragon, without the intervention of Gorbash, and becomes Carolinus' apprentice to control this ability. Having learnt this control enables several extraordinary deeds in the service of England, and later makes Jim eligible to join the 'Collegiate of Magickians' whereof Carolinus is a founder (a subplot emphasised in later books).

...
Wikipedia