For 121 upgrade, the best thing you can do is careful and exhaustive observation of the captains with whom you fly. See what they do. See what they don't do. Find things that you like, don't like, and begin to incorporate these observations in your own actions and plans. Note their cadence, their planning. Take the time to sit down over dinner and discuss their thoughts. Ask them about their day and preparing for a trip. Paperwork. Following a trip.
Far more useful than a book will be the first-hand examples that surround you.