I'm wondering about the following question: if I want a 12 inch wide board from a maple tree, say, how wide a tree would be needed so that after you cut off the bark and the sapwood, there's still 12 inches of heartwood left? Are there guides for this kind of thing, by species, or a general guideline for all trees, or is it too variable to give any general rules?
For Maple and similar hardwoods (oak, ash, beech, chestnut, walnut etc...etc...) you will find that there is very little sap wood vs heart wood. And what little sap wood there is (example walnut) its still good and hard enough to keep in the board for whatever reason.
As a woodworker myself, and I mill alot of my own wood via Alaskan Saw mill or repurposed wood, sapwood is not discarded or removed. I attempt to use all portions of the wood as I can, depending the project. And even then, I have a tendency to keep my scrap to reuse in another project, or a jig, or in my lathe or if all else fails I use it in my smoker or as firewood for winter.
But, if you are looking for a good book explaining different types of wood and their uses, look for "The Essential Wood Book: the woodworker's Guide to choosing and using lumber" edited by Tim Snyder.
To add to Kyle’s excellent answer, if you are looking for heartwood specifically for the different coloration (though you mention maple, which is fairly white regardless), then you will lose an inch or three of each board. Here’s a cross section of a birch slab I milled recently. As you can see, the entire width is usable wood, but the darker heartwood is only about 6-7” of the total 10” width. In the tree, sapwood has more tensile strength, heartwood more compressive. Once you mill the lumber, the dried board is pretty much all the same. Even commercial mills simply take off only enough slab to get the cant straight enough for milling into boards. Many only debark the logs, then saw the lumber which then goes to an edger to become a straight (parallel edges) board. No effort is made to separate sapwood from heartwood.