Memory, by Lois McMaster Bujold

In anticipation of Bujold's new novel in the Vorkosigan saga, I've been re-reading some of the old ones in my library. Memory is the novel in the series that marks a real turning point in Miles' life. He's been working for ImpSec's Galactic Affairs office undercover as the wily and unpredictable Admiral Naismith of the Dendarii Free Mercenaries for a number of years, quite successfully, and now a new assignment goes horribly wrong. While rescuing a Barrayaran courier from kidnappers, Miles suffers a seizure caused by some lasting effects of his cryo-revival the previous year, and while he's seizing, his plasma arc locks on and severs the legs of the courier (fortunately, the medical people can put Humpty back together again).
This will not look well in his report to Simon Illyan, chief of ImpSec, and Miles makes the crucial error of deciding to falsify the report, making it seem as if it was a weapons malfunction that caused the mishap. He knows that if Simon knew that he was having seizures, he'd pull him off of duty with the Dendarii and put him on a desk job. As one would expect, Simon discovers the deception, and Miles is forced to resign from ImpSec for "medical reasons", rather than face a court martial for his deceptions. So much of Miles' self-image is caught up in his success in ImpSec and in playing Admiral Naismith that he is absolutely lost, rudderless, and near suicidal at this point.
Fortunately for him, there are other plots afoot. Simon Illyan has had an eiditic memory chip in his brain for most of his life - and all of his ImpSec career - that has made him a formidable weapon in the Emperor's arsenal. Someone finds a way to destroy the memory chip, and nearly destroy Simon in the process. Despite their recent contretemps, Miles feels a great deal of affection for Simon, and a duty to him as well, as a loyal satellite of the Vorkosigan clan, a sort of adopted uncle, and tries to visit him in the secure hospital where he's being held, slowly going insane due to the misfiring and deterioration of the memory chip. Frustrated when he can't get in to see Ivan, Miles plays his trump card by visiting his foster brother Gregor, Emperor of Barrayar, and wangling an Imperial Auditor's position, to investigate the attack on Simon.
This story has a great mystery as its heart, with Miles deftly unraveling the strings, and some fantastic growing moments, as he finally puts away "childish things" and reconciles himself to his Barrayaran destiny.