Allusion in Friar Brackley's Sermon

In Fenn's *Paston Letters*, XCVIII. (vol. iii., p. 393., or vol. i., p. 113. Bohn), entitled "An ancient Whitsunday Sermon, preached by Friar Brackley (whose hand it is). At the Friers Minors Church in Norwich" occurs the following:— "Semiplenum gaudium est quando quis in præsenti gaudet et tunc cogitans de futuris dolet; ut in quodam libro Græco, &c." "Quidam Rex Græciæ, &c.; here ye may see but half a joy; who should joy in this world if he remembered him of the pains of the other world?" What is the Greek Book, and who is the king of Greece alluded to?