Do you have any (unique) advice?

I do, though I would never describe my skills as superlative or any similar case.

@Winterwanderer Practice makes perfect! You write well, but can also your develop your skills of writing and observation. So, study/get writing

But how could I improve my inspiration & motivation to write a book-length of material?

And how would I do everything else?

@Winterwanderer Don't wait for inspiration, motivation. Get going, action! These will follow. Learn more about writing. Write daily

Sounds like a romantic idea, I might just write more because of that.

@Winterwanderer Good :)
You'll enjoy it I think
Good luck!