By all accounts, it would appear to be the mythical Jason (of the argonauts fame!) if you are counting large books, rather than pamphlets.

This was printed by William Caxton when he returned from France, having seen the first Engilsh language book published at Bruges.

The first products of the Westminster press were small pamphlets. Now this description exactly applies to a number of tracts of small size issued about this time. These are Lydgate’s Temple of Flass, two editions of The Horse, the Sheep and the Goose and The Churl and the Bird; two editions of Burgh’s Cato, Chaucer’s Anelida and Arcite and The Temple of Brass, the Book of Courtesy and the Stans puer ad mensam. From what we know of Caxton’s tastes, these are just such books as he would be anxious to issue. The first two large books which he printed were The History of Jason was translated by Caxton from the French version of Raoul le Fevre, and undertaken immediately he had finished the Recuyell of the Histories of Troy and The Game of Chess.

Lots more info here:
http://www.bartleby.com/212/1303.html

I think Duffield is correct and the Recuyell of the Histories of Troy and The Game of Chess were printed by Caxton in Belgium. This is from the University of Glasgow:

“The first book Caxton printed, and the first book to appear in English, was his own translation of the History of Troy. It probably appeared in late 1473 or early 1474. In all he printed six or seven volumes before returning to England; these bear no place or date of printing but were almost certainly printed at Bruges”.

http://special.lib.gla.ac.uk/exhibns/printing/

If you look further back in the link given by Duffield, it does say this in the chronological order of events:

“Caxton, in his Game and playe of the Chesse, made use of both these versions, translating partly from one and partly from the other. The last book he printed at Bruges was the Quatre dernieres choses”.

http://www.bartleby.com/212/1303.html

This is confirmed by this article from the New York Times of 26 July 1902 which describes the purchase by Morgan of a private book collection containing twenty-two Caxton’ including … The “Chess Book” of J. de Cessolis, printed by Caxton (with Mansion’s aid) at Bruges about 1575.

http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9A0DE6DB113DEE32A2575...