which of the books would you recommend for someone who watched and enjoy both anime seasons but hasn't read any of the book series'

I’d recommend starting with the very first one, Sword art online 001: Aincrad

The novels cover the events of the series in way more detail than the anime. More character motivations, more development, more world building and game mechanics. They’re a very different experience.

If you want to jump in right where the anime left off and where season 3 will start, then start with Sword art online 009: Alicization Beginning.

If you are like many a huge fan of the first arc of the series, Reki Kawahara has started a full rewrite of the Aincrad arc. Sword Art Online: Progressive 001