I think there were about nine novels that came out during the series' run. Here's a site with information about most of them:
http://vicki_98.tripod.com/novels/index.htm
A. C. Crispin's novelization, as stated, adapted both miniseries; it was a very thick book for the time. The only other books to deal with the main cast of the series were
Prisoners and Pawns by Howard Weinstein and
Death Tide by Crispin and Deborah A. Marshall, though a few of the other books have guest appearances by series characters. Two books,
East Coast Crisis by Weinstein & Crispin and
Path to Conquest by Weinstein, featured the New York resistance; ECC covered the same timespan as the two miniseries, telling a parallel story, while PtC took place during the series.
Basically the Crispin and Weinstein books are the only "essential" ones. The rest take place in various other parts of the US and elsewhere and feature mostly original characters, though a few have cameos or guest spots by members of the TV cast (Ham Tyler has a prominent role in
The Florida Project and Willie does in
The New England Resistance). They're mostly pretty mediocre, although the two by Somtow Sucharitkul (an author perhaps better known today as S. M. Somtow),
The Alien Swordmaster and
Symphony of Terror, are fairly interesting (though if my vague recollection is anything to go by, I don't think the latter was as good as the former). IIRC, both the Sucharitkul books focused on the same protagonists, making them the only ones outside the Crispin & Weinstein books to feature recurring characters. Several other authors did two or more books in the series, but they all featured different characters and locations.
