This is my version of a slightly invisible bookshelf. There is a chance of seeing the bracket under the books. It cost me less than $5 for the materials (not including the tools). Each bracket cost me $1.78 and the screws were $1.24 for a pack of 18.

Step 1: Gather the Materials

Materials:
3" x 7" Strap Tie at Lowes, $1.78 per strap

#6 x 5/8" Sheet Metal Screws (5 per bracket works great), $1.24 for 18pcs

Tools:
Screwdriver for the screws
Something to bend the metal (clamps and mallet but a vise works great too)
Level

Note: the last 3 pictures show them already bent. They are sold flat, not bent. So don't expect to find them at Lowes already bent, like they are in the pictures.

Step 2: Make the Brackets

Bend the metal by:
1) aligning the 3 holes closest to the middle on the edge of a table (or in a vice)
2) secure the metal to the table with clamps
3) bend the metal by hand or with a mallet to form a 90 degree angle (80 degrees or so for heavy books).

I've attached a video to show you guys exactly how I bent it.

Step 3: Attach to the Wall

The long end will hold the books while the short end will be secured to the wall. Secure the brackets to the wall with screws. Ensure the shelves are level and evenly spaced. Add shelves as needed.