Just read 'Waiting Games'

Twilapickle, the above poster is mistaken, there are only three novels in the series: Sooner Or Later, Waiting Games, and Now Or Never. Bruce and Carole Hart wrote another rock-themed novel set in NJ called Strut, but it is unconnected to the Sooner Or Later series.

I bought a copy of Now Or Never back when it was still in print, and I can tell you that the story does reach a conclusion in that book, even though the Harts left themselves a bit of room to keep going just in case. I'm not quite sure why so many people feel that the story was left unfinished. My guess is that the main reason the book was written decades after the second one was to finish what they'd started, not really to revive/reboot the series to go on and on. It's worth noting that there was something like 20 years between the publication of Waiting Games and Now Or Never; based on the fact that the book seems to have been out of print ever since its initial run, I'm guessing that whatever success it had didn't warrant continuing, anyway. The Harts wrote a string of YA novels for Avon Flare in the late 80s/early 90s, and Now Or Never was the second to last of them.

As for Carole Hart writing another book without her husband - her most recent work seems to be a documentary released in 2009 called For The Next Seven Generations. In addition to her husband's death, she had her own serious bout with cancer in the 1990s, which seems to have turned her to a more spiritual/global bent; in other words, I wouldn't hold my breath waiting for her to write more Jessie and Michael books. However, it's too bad no one seems to have hipped her to how much money she could make offering all three books in e-book form. Why let these third-party sellers make all the cash off of her work, the rights to which have very probably reverted to her by now? Or if Avon still holds the rights to the books, why are THEY letting others make all the money?