Quick and dirty FAQ:
"How should I watch it" release order. Seasons 1 & 2, FWWM, Missing Pieces, Season 3.
"Season 2 is really long" there's a quality drop at a certain point midway through. It's really obvious when it happens. At that point, you can fast forward through some of the more boring subplots without missing anything important. Keep watching cause it gets good again and the last episode of season 2 is the best episode of the original run.
"Are the books important" they're fun but no not at all.
"This show's old, does it hold up" The movies and season 3 absolutely do. Season 1 and a large chunk of season 2 also do, assuming you keep in mind two things: This is far before the golden age of television, so some of it may feel a little restricted in comparison to the amazing things modern tv pulls off. The first two seasons are a parody of soap operas that used to occupy its timeslot, so a lot of the cheese is very much intentional.
"I've never seen the missing pieces" It's a collection of deleted scenes from FWWM editted into a movie. Some plotpoints arguably come up in season 3, and some scenes in FWWM are made a little less ambiguous through it. You can watch season 3 without seeing it though.