All right. Time for me to weigh in.
Bearing in mind that the first two clues are going to be the most useful, we have Severe Injury and Bookstore. Now, Severe Injury is rather an unfortunate clue, as it's something of a "Miscellaneous" category--in other words, it encompasses those modes of death that don't fit in any of the others. Typically I associate it with trauma by blunt instrument, as cutting/piercing weapons can be described by "Loss of Blood." But it could also cover other modes of death, such as Leslie's Explosives.
Bookstore is a better clue, as it's highly specific. Based on this clue, we can eliminate myself, MP, Rel, Christa, Ryven, and Phoenix, as none of the items they have are associated specifically with a bookstore. That leaves NightOwl, Yugi, Rapt, and Leslie as potential suspects. I'll go through them one by one.
NightOwl - His Gift would be the obvious choice here, although, as Madame Professor pointed out, the Summer clue kind of shatters that theory. If I were Blythe trying to point attention to the Gift, I would most definitely have said Winter. As for his weapons, either Whip or Stone would be viable candidates, although I'm not sure that Disorderly really points to either.
Yugi - Although unlikely, it's possible that maybe Blythe was trying to point us toward either the Scissors or the Trophy with the Bookstore? Although, on second thought, I probably would have said School for the Scissors, or some kind of athletic structure for the Trophy (gym, arena), so this is a longshot possibility.
Rapt - When I saw Cassette Tape, I immediately thought "Books on tape". Her Kick, Folding Chair, or maybe her Hook could all qualify as causing Severe Injury. Unfortunately, Summer (unless Blythe happened to be referencing the Bryan Adams song, "Summer of '69") doesn't really help us here, nor does Disorderly.
Leslie - Her Notebook is probably the #1 associated clue with Bookstore, and if that's what Blythe was trying to get us to focus on, Bookstore was a very good clue indeed. It even has "book" in the word. So that leaves us with her weapons. Ice Skates would either qualify as "Loss of Blood" or "Accident"; a Metal Chain would most likely be used as "Suffocation"; that leaves us Explosives and Unarmed. Since MP incorrectly guessed Unarmed and Notebook--an excellent guess, I thought--that leaves only Explosives / Notebook. However, if Explosives was the manner of death, he surely would have said "Bits and Pieces" for clue #3. I'm not sure what Summer was meant to do, other than eliminate Ice Skates as a possibility.
At this point, I feel like Blythe has eliminated every potential combination, so I'll wait for more information before assessing further.