Generally timber (whole trees) is rough cut into 1 or 1-1/2 inch thick pieces of lumber, 5 or 10 feet long, and as wide as the actual timber. It is stacked flat, the individual pieces separated by small spacers of wood (called stickers), and allowed to dry for 6 months to a year. Then it is planed into dimensional lumber, like the boards you see at Home Depot.
Just guessing, but TheSlightestGinge's grandfather probably had a custom timber-cutting crew come to his yard, rough-saw the fallen tree, and stack it near his woodshop. That's what I would do. Rough-cutting timber takes different equipment than typical woodshop gear--think sawmill-type machinery.
An excellent reference is Earnest Joyce's <em>Encyclopedia of Furniture Making</em>.
https://lostartpress.com/collections/books/products/the-essential-woodworker
https://www.amazon.com/Encyclopedia-Furniture-Making-Ernest-Joyce/dp/0806971428
If you also search Google Book that are free, Griffin and others will show up for free as they are out of copyright.
I'd start here:
This was one of my favorite reference books that lists all sorts of designs and background. Once your comfortable with what you see and feel you can accomplish a specific joint, design a project around what skill you want to hone.
The Walmart bookshelf looks nice and if you decided to build something similar, you could use ply with solid wood legs raddited into the edge. The final look could be done with a piece of hardwood attached around the entire boarder of the shelf and leg. Taking it a step further, try bread board edges and all hardwood shelves. Each step allows you to get a tool to make building easier. A table saw and/or router would help start you off.