Reddit mentions of Vertical Mind: Psychological Approaches for Optimal Rock Climbing by Don McGrath and Jeff Elison (2014) Paperback

As far as improving your headgame goes, leading easy but long runouts is super helpful, as well as falling onto (well-placed) gear. Alpine multipitch is an admirable goal, but it is a far cry from what most people think of as trad climbing. You should be confident on long runouts, with potential no-fall zones. There are a lot of skills that are important for alpine climbing that often are not learned in a typical trad climbing mentor relationship, such as self-rescue, alpine route finding, and depending on your goals, snow climbing skills. There are several ways to learn these skills such as books or hiring a guide. Ultimately, your safety is much more dependent on yourself when alpine climbing. I say this not to scare you away from alpine climbing, as it has been responsible for some of the most amazing memories I have, but it has also been responsible for some of the scariest.

Some books that you might find beneficial:

Climbing Self-Rescue - Just what it says in the title.

Vertical Mind - I found this book useful for improving my head space.

Training for the New Alpinism - Probably the best book to help a climber transition into the backcountry.

[Mountaineering: The Freedom of the Hills] (https://www.amazon.com/Mountaineering-Freedom-Hills-Mountaineers/dp/1680510045/ref=sr_1_1?keywords=freedom+of+the+hills&qid=1562736585&s=gateway&sr=8-1) - This book is excellent, but probably isn't extremely helpful until you are climbing more serious alpine routes.

As far as advice, just get as much mileage on lead outdoors as you can, with 1-2 indoor bouldering sessions per week. If it doesn't impact your bouldering, you could add a couple strength sessions as well. If you want to get into alpine climbing, or even just multipitch climbing, practice your systems at the top of single pitch routes. Belay your partner from the top, practice building an anchor at the top off of the bolts, set up simple pulley systems. Just spending 15 minutes per session will help you get muscle memory down for when it really matters.

Edit: As you get into more alpine climbing, you should increase the strength training and cardio. Climbing efficiently after four hours walking with a pack full of gear and food is harder than it sounds. Increasing your physical strength will reduce the mental load a lot, allowing you to think more clearly and be more confident.