Learning about cocktails must be experiential, but it doesn't hurt to know about cocktails, spirits, and ingredients that shape these drinks. Here are some of the best books I have read on the subject.
Boozehound was a great book that really helped me get into quite a few different types of cocktails. Jason Wilson does a great job of introducing all types of spirits and providing some fantastic cocktails. My best takeaways from this book have been getting Old Tom Gin (Ransom's) for the Martinez and the Fourth Degree, and trying various Campari cocktails, my favorite being the Boulevardier.
All drinking enthusiasts have a connection to Hemingway. This is a great book on his life and the cocktails that showed up in his stories and his life. His daiquiri is fantastic.
Amazing book. These guys at Death & Co. really know what they are doing and they share a lot of knowledge with everyone in how they lead the charge in the cocktail world.
A more academic read on prohibition cocktails from the famed Jerry Thomas, but filled with so much history and knowledge that you feel you know the era and the players that made America the king of cocktails.
I picked up the Drunken Botanist at Catoctin Creek Distillery when Emily recommended it to me. She was very helpful and knowing the qualities of the ingredients that go into the spirits can really help identify flavor profiles and just help you understand the complexities that go into them.
A wikipedia of knowledge about all things Whisky. I hope you like Scotch.