In addition to the two existing good answers, I'd like to add that the fuller version of "guilty" is "guilty as charged". The basic meaning of this phrase is the same as "guilty" as explained by Michael Harvey and jla, namely, to affirm that one is responsible for a crime that they have committed.
The state will prove that the defendants are guilty as charged. (Merriam Webster)
But there is a figurative sense that is commonly used colloquially. See Cambridge Dictionary
used to admit that what someone has been accused of is true, often when you think this is not really bad:
Guilty as charged! I am an Elvis fan!
Yes, he's guilty as charged of being a show-off, but that's why he's so entertaining.
This has a slight implication that the thing one is being identified as can be jocularly likened to a minor offense. "You like Elvis Presley?" "Guilty!" This basically means: "Yes I am an Elvis fan. And I am proud of being one." Of course being an Elvis fan is nothing to be ashamed of, but the funny part is you are using "guilty" to imply that some people might think that, (but you are proud).
I found your source on Google Books. Fuller context:
But even before he turns around, I know him. I had no idea he worked here now, or else I wouldn't have come inside. I should have arranged for a taxi before I arrived, or better yet, rented a car in Glasgow. Now, it's too late. He turns around and spots me. Byron.
Apparently "I" do not want to see this Byron guy and "I" didn't know he would be here, otherwise "I" wouldn't have come. Of course there are reasons why that is the case, and you will have to get it from the prior passages, but the situation described here is clear. "I" feel embarrassed and thus reluctant to see Byron, and that is implied in the use of the phrase "guilty" as a way to say "yes" (give an affirmative answer) to "It's you, isn't it!"