I've got 3 built-ins. I built 2 and my wife and her mother built one (they aren't carpenters). (I've since refined that one a bit) We've discussed removing it and starting from scratch, but there are other projects to be done before that happens.
The big problem that you may have with trying to make the built in is that the wall/drywall may not be completely flat, or maybe ours wasn't because it's 40 years old and has lived in hot, humid Houston all it's life. What that means is that there is a gap in some spots at the back edge of the shelves, but I never really notice it. I built my shelves to be square to each other. They built theirs to fit to the wall, so it didn't turn out quite the same. That's why I've worked on it a bit here and there.
All three are built using 1"x12" pine (starting with 6' or 8' lengths depending). I used "premium", they used the cheap stuff. The premium is better because of the lack of knot holes and it doesn't seem to sag like the boards with the knot holes.
The one that my wife built is somewhat like the one that you're describing, I think.
They made a base of 1"x3" pine to stand the thing off of the floor and then put a base of the 1x12 on top of that. They then attached a 1x12 to the wall on the right and left. They used 1/2" x 3/4" strips to support the ends of the roughly 36" shelves. It's a 12' wall so they added 3 vertical supports each with the same 1/2" x 3/4" supports. It holds a ton of books, and the shelves don't sag (I've replaced about 3 or 4 saggy boards with better quality pine), but it's not exactly pretty. 3' shelves are about as long as you could go for a shelf that is only supported on the ends. If you've got a support running along the back, that would be better, but I'm not sure if the front would sag due to the weight if the shelf was 4' or more.
I built mine by building a box and then filling the box in. I used a dado blade and/or a dado router bit to put 3/4" grooves in the vertical sections to support the ends of the shelves. I also have vertical supports. My bookshelves are REALLY strong.
This was my first project of this type/magnitude. We've since painted it, and it looks much better. I used putty to cover the screw heads. It was a paintable putty, but not a stainable. I've since learned the difference.
This is not really the same. We had this enormous fireplace that we didn't use that took up most of a whole wall. I turned it into our entertainment center. I built it on the fireplace and measured each board to fit it's spot because nothing is ever quite square. The thing is not actually attached to anything, but it also is completely immobile and won't shift if you try to shake it. I built a box with a door that goes bottom center. The Bluray player, cable box, etc... goes in the box. The TV goes above that. I was going to mount it to the brick but settled on it just sitting on the shelf. The fireplace is essentially completely hidden by those two things. I almost forgot, I built the shelves to the right of the fireplace too.
You probably could make one with the supports and still have it turn out nice. It would be much easier if the wall was nice and flat. You'd probably have to make it interesting. Maybe use the same types of supports to look like they are supporting the vertical boards as well.
I'd probably go with something a bit different, but then, I'm very much an amateur at this sort of thing. There are some really impressive guys on the board that would put my creations to shame.