Reddit mentions of The Pathwalker's Guide to the Nine Worlds

Start with Neolithic Shamanism by Kaldera and Krasskova

Then start Kaldera's Northern Tradition series:

Jotunbok

Pathwalkers's Guide


Wightridden

Wyrdwalkers

In the meantime, start searching for the Galdrbok by Johnson and Wallis and read that when you can get a copy.

That should give you about 3-5 years worth of training material. Once you have worked through it all, set it aside and focus on how the spirits you work with encourage you to practice. All of the above authors have their own practices and much of what they offer will not be completely true for your work. Learn the skills that are presented, make them your own, and then foster your relationships with the spirits from there.