You have a couple of options when making wall-mounted bookshelves. You can build a box-style shelf and mount it to the wall, or you can make a shelf that has no visible means of support for a clean and classic look.
Decide how big you want the box shelf to be. Work in full numbers if you're new to woodworking. For a simple shelf, use two pieces of wood cut to 2 feet wide and 4 feet long.
Add two end pieces that are 2 feet square and use glue and screws to secure them together.
Cut a section of plywood and attach it to the frame with screws. Screw through the back so the screws won't show. Screw through the back into studs for a simple wall mounted shelf.
Add any embellishments you like to the shelf by using crown molding, paint or stain to enhance its appearance.
Build a hollow box. Make it as wide and long as you like and use glue and screws to secure it together.
Attach a length of board cut to fit the hollow of the box you built in Step 1. Screw them into the studs at an angle. The appearance of this won't matter, as it won't be seen.
Slide the box over the wood attached to the stud and screw it into place from the bottom.
Countersink the screws and fill them with wood putty. Touch up the paint or stain once the putty is dry.
Embellish your wall mounted bookshelf with crown molding, dental molding, or by routing the edges for a more decorative look.
You can make the shelves described above any size you like. Reinforce the box-shaped shelf with supports just like the end walls if you make it very long. This creates compartments that provide strength and help avoid bowing. Sketch out the dimensions of the wall-mounted bookshelves you want to build and take it with you to your local home improvement store. They can cut the wood to size for you, making your construction much easier.