There are multiple books on how to use them. Them and every other instruments. It's called instrumentation or orchestration, a lot of famous composer wrote books about that, from berlioz to koechlin to adler.
i am currently reading the one by Berlioz : https://www.amazon.fr/Treatise-Instrumentation-Hector-Berlioz/dp/0486269035/ref=sr_1_3?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=de+l%27instrumentation+berlioz&qid=1610784557&sr=8-3 (i'm not sure it's exactly this one)
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This one by adler is expensive but it's a bible for orchestrator : https://www.amazon.fr/Etude-lOrchestration-Adler-Samuel/dp/B0054UKJAM/ref=sr_1_1?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=adler+trait%C3%A9+orchestration&qid=1610784724&sr=8-1
(they are not written specificly for strings, but for every instruments)
Berlioz/Strauss' Treatise on Instrumentation is another good one.
Berlioz wrote a very famous orchestration book. Richard Strauss loved it and later added his own insights to the text. Dover published them together. Can't deny the writings of two master composers.