Could this be the series Countdown?
According to the Wikipedia description:
New Year's Day 1999 brings a strange solar flare, widescale power
outages, and the death of everyone on Earth over twenty years old,
leaving desperate teenagers to face the awakened Demon called Lilith.
As the new year dawns, a reported "massive solar flare" causes power
failures all over the globe and adults and children everywhere to melt
into piles of "black goo." Only young adults are spared, among them a
quartet of drunken high school kids in suburban Seattle, two teens
whose fake IDs have gained them entry to a New York City nightclub, a
cocky young doctor in a Texas hospital, a pair of tough-talking
inmates in a Pittsburgh jail, and Sarah and Joshua Levy who
desperately search the ancient scroll of their granduncle Elijah for
clues to the apocalyptic event.
They all seem to have been released in 1999, with the unusual gimmick of each being released one month before its titular period (the books are named after months).