In The Schizoid Man, Ira Graves, who had previously hijacked Data's body, transferred himself into Data's computer. I know this is never spoken of again in the canon, but is there any expanded universe reference to what happened to the... being of Ira Graves after this point?
Frankly, not a lot. Remember that Data and Picard were very clear that although Graves' knowledge was transferred to the computer core, his sentience (without a positronic matrix in which to house it) was lost forever.
Picard : The intellect of Ira Graves has been deposited into our computer. There is knowledge but no consciousness. The human equation has been lost.
The original show script was even clearer on the issue :
KAREEN : (sad half-whisper) He's in there. Ira put himself in the computer.
PICARD : No Kareen. The real Ira Graves never left the planet.
This information placed into the computers is referred to (very briefly) in one of the Trek novels;
Maddox was much calmer as he answered. “There is a wealth of information available on all the other Soong androids, including Data, Lore, and Lal. We have the information recovered by the Enterprise from Graves World after Ira Graves’s death, as well as information received from the studies of AIs over the past two centuries.” Articles of the Federation