It should be noted that the anime Boogiepop wa Warawanai is actually a different story than the Boogiepop wa Warawanai that appears in the novel, manga and live-action movie of the same name. (For the sake of brevity, the title will be shortened to just Boogiepop from now on.)
The original Boogiepop story was told in a very similiar fashion to the anime -- it was the story of a particular chain of events told from the perspective of several people somehow connected to it. (In the anime, this 'focus' was the beam of light event.) Without giving too much away, the focus of the original Boogiepop was a monster called Manticore. Manticore took over the body of an unfortunate girl and hid in the highschool it found her at, feeding on people unfortunate enough to find her. One day, a rather psychopathic student called Masami Saotome runs into her -- and falls in love. At the same time, a mysterious man called Echoes shows up in the city, and due to his appearance is shunned by everyone except one kind girl...
Fans of the anime should be very familiar with the names "Manticore", "Masami Saotome" and "Echoes", but may be left somewhat confused by the fact that the anime doesn't say a hell of a lot about them. This is because the anime is really aimed at people who know the original story quite well. In other words, a large part of the mystery and confusion of Boogiepop's story for Western audiences is due to the fact that they never saw the first installment of the story. After watching Boogiepop the anime after I'd managed to read through the manga, I saw the series in a very different light -- and understood it much better.
There are quite a few other Boogiepop stories by Kouhei Kadono, and there is also a spin-off manga called Boogiepop Dual, which as far as I can tell is just a one-off story which has nothing to do with anything.