Is this the cover in the painting?
Certainly a lot of people seem to think so and some things do fit. Titus Groan is, of course, the first book in the Gormenghast trilogy by Mervyn Peake, and this particular Penguin Modern Classics edition was, I believe, first published in 1974. The drawing on the cover (by Mervyn Peake?) is also somewhat similar stylistically to the work of Balthus, mentioned in the title of the painting, so that would make sense I suppose.
However, the features of the faces do not look particularly alike — the features in the painting are more angular — and the hair/shadow to the left of the face on the book cover is notably absent on the cover in the painting. The mood of both seem quite different (to me at least). Can this simply down to artistic license or painterly technique on the part of Hagan?
Other compelling suggestions have been thin on the ground. T.E. Lawrence’s The Mint has been suggested, and there are some similarities to the cover of The Waves by Virginia Woolf, but neither seems quite right and they do not fit with the trilogy alluded to in the title.
There may never be a definitive answer — the artist, Frederick Hagan, died in 2003 — but please let me know if you have any further suggestions or thoughts.