As for knowing his mother, why should he be any different than anyone else. You know your mother because she was there as you were raised. And at that you know your mother, not the person she was. If she had died before you were born, she would be a collection of photographs and stories told to you by the people who knew her. And look at who Harry's mother ran with. A bunch are dead, or inhuman. The ability to research implies a record to examine. In a lot of ways Margaret was an International Women Of Mystery.
Knowledge was also deliberately kept from him. His mother died when he was born, his father, when he was six, instead of stepping forward and claiming him, his maternal grandfather let him be
dumped in an orphanage for the next four years with no clue of anything except vague memories of his loving father in the old station wagon they traveled the country in. When he was adopted by Justin he had no reason to believe he had any living family. What little he knew of his mother is what he was told by his adoptive father, whom he loved and trusted until he was sixteen. Then the
crap hit the fan, his fae god mother didn't or couldn't tell him the truth... When he was saved at the
trial by Eb, he basically learned nothing about his mother other than she was a killer rebel who
was under sentence of death. Eb then took him in for the next three years, never even hinting that he was really his grandfather... Later in Blood Rite when a wounded Harry confronts him about a lot of things, Eb still couldn't or wouldn't tell him the whole truth about who he was or his mother.. Harry was shocked and pissed about what had been kept from him, but really had no basis to probe further.. He finds out from Thomas that they are half brothers in the same book... He sees and talks with his mother in a soul gaze but beyond confirming that he and Thomas are indeed brothers she gives him very little information save she was sorry about the future she set him up for... Thomas is no real help either, because he was also a small child when she ran away.. He gets bits and pieces, hints about his mother, but they are fleeting like what Chancy told him.. But the price of getting the full story from him is was way too dear... Lash promised to tell him everything she knows, but then she was killed.. But you have to ask, why didn't she volunteer this information sooner to him? Finally in Cold Days Rashid finally sees fit to tell Harry that he was good friends with his mother, that they had dinner together and swapped information about the ways, but it wasn't the time to probe futher... There is a hint of a conspiracy, one to keep Harry as safe as possible, the other to keep information from him as long as possible until needed... One having a lot to do with the other.. Research into what happened to his mother? It isn't like Harry hasn't been a very busy man the last thirty or so years.. He cannot simply "google" what he wants to know and when he wants it.. Those who do have information kept that they ever knew his mother at all from him until it oozes out, but then they keep most of what they know from him... He really hasn't had the luxury of time to obsess about it..