I would keep buying white sifted bread flour from a normal source, and do home milling for the flour where you work *with* the strengths of your home mill, not against it. That is - when you want fresh wholemeal flour. Grains and seeds in general are nature's own way of *best* preserving nutrients intact for a long time, the moment it gets ground it starts aging faster. There's something extraordinary about the freshness and the taste you get from freshly ground wholemeal flour. You also get full control over texture / coarseness.
It's very hard to mill and process sifted high-strength bread flour at home. It often takes multiple milling steps and siftings, and you are dependent on how fine your mill can mill. Mills with natural or ceramic grindstones have an advantage in that they can go finer and produce flour with a different texture than mills with steel burrs. But even then, it is an awful lot of work.
I would keep buying white sifted bread flour from a normal source, and do home milling for the flour where you work *with* the strengths of your home mill, not against it. That is - when you want fresh wholemeal flour. Grains and seeds in general are nature's own way of *best* preserving nutrients intact for a long time, the moment it gets ground it starts aging faster. There's something extraordinary about the freshness and the taste you get from freshly ground wholemeal flour. You also get full control over texture / courseness.
Since you have a mill with steel burrs you should be able to successfully mill even non-traditional grains. Some oily grains clog up stone mills but process fine in a mill with steel burrs. Think flaxseed, quinoa, sorghum & others.
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Now what grains to look for?
I'd start with two basics that really leads to fantastic bread. Rye (heirloom if you can get it), and Emmer.
Rye is great overall, nutrient-rich and tasty - and has lots of attractive qualities that make it a good idea in bread. It's also a really good flour to keep and maintain sourdough starters with. Even in breads where you don't want much rye taste, starting with a rye sourdough and/or adding a small amount of wholemeal rye is a great idea and pretty common.
Emmer is something else. Bread with freshly ground emmer are incredibly tasty. It takes a bit more care when baking, but the results are so totally worth it.
Einkorn and barley are both sorta neat. Einkorn is harder to bake with than the other wheat-related grains. Barley is neat but a tad boring. (You *need* to look for naked/hull-less heirloom barley if you intend to mill it yourself.)
Wheat? Shop around locally, try - taste - and experiment. It is a staple in bread making for a reason, and if you do wholemeal wheat flour, the differences between varieties can have a noticeable impact. Personally I don't bother with wholemeal wheat, and the only wheat that ends up in my bread is storebought sifted white wheat. The differences are smaller when it is sifted, and more about technical baking properties than about taste.
I'm neutral and indifferent of spelt. Never felt a need to try it, I got enough grains to draw from already for my bread. I know others like it and have success with it.
Here's a link with some basics on rye:
https://www.thefreshloaf.com/handbook/rye-flour
Here's a good blog post on emmer:
https://www.mygermantable.com/what-is-emmer-wheat-and-how-to-bake-bread-with-it/
As for books? I'd want to have at least one high quality reference work for all the normal bread everythings *with* technical background and info that most books skip over...
Something like Peter Reinhart's Bread Baker's Apprentice.
You probably also want a more focused book on whole grain breads. I can't vouch for it personally - haven't read or worked with it yet but Peter Reinhart's Whole Grain Breads is probably a good pick:
Last, I'll throw in a recommendation for Dan Lepard's The Handmade Loaf. It is a whirlwind of interesting bread varieties from all over the world and worth a read just for inspiration's sake:
https://www.amazon.com/Handmade-Loaf-started-baking-revolution/dp/1784724424/ref=sr_1_1?