Made a lot of mistakes, learned a bunch.
Embrace your mistakes and learn from them, but don't tell anyone about them. 99% of the people won't know any better.
That is why actually building things is very important. woodworking is a skill not a thought exercise in school. You can memorize all the magazine articles in the world, and have all the answers, until you make something with your hands, you have'nt done anything.
If you want a specific critique on design and such here goes. keep in mind that my own taste are different than someone elses.
Fill it up with books then post a picture. Book shelves with nothing on them never look right to my eye. People see the books and never notice the book shelf
the 2x10 southern yellow pine won't sag over that span. A major plus in my mind
the short end shelves, and top. I'd think about making those a 1/4 of a circle. I have 2 reasons, one is pure aesthetics and the other is practical. Walking by the bookshelf, that sharp corner is likly to get bumped into.
the cove in the walnut leaps out to my eye, it looks large to me. (again no books so its hard to really tell.) I'd probbaly use a small round over profile, though a smaller cove could look pretty sharp.