All of the John Shewey books are awesome, but probably the best for someone newer would be “Steelhead Flies” as there is more instructional info. The classics book is also great though, and his spey and dee flies ... lots of good stuff.
As for your leader questions, a 12’ leader isn’t that big of a deal on a 13’ rod (especially with modest sized flies summer/fall flies). For most of the rivers around here I tend to find there is a much bigger difference for “Winter” after about mid December, than there is between “summer” and “fall” fish. Just my personal taste and experiences, but I fish pretty similar equipment and smaller flies all the way through Thanksgiving (or later depending on water temps). Winter is a whole other deal entirely, with sink tips being pretty much standard.
"Lord help me to be the person my dog thinks I am"