How do I turn old flooring into a bookshelf?

You could use the flooring as shelves and then use cinder blocks to hold them up. That's a pretty simple option :-)

My mom used these to build an entire wall of shelves. She turns 70 this year !

https://www.knapeandvogt.com/product-page/adjustable-shelving

I am building this.. but it will be 8 ft tall and 5 1/2 foot wide. It will be in the place of an entertainment system that is being removed.

I went to Lowes and found the pieces I needed... (they do all the cutting and threading for free.)

Two 10 ft. black iron pipes cut into 16 pieces, threaded on both sides, 3/4 inch wide

4 floor flanges

16 couplings

16 large washers

a hole saw.

all 3/4 inch.

I have alot of books and its going to be heavy.. so I decided on the 3/4 inch gas pipe. I will not be using the t's or fasten anything to the walls.

I am going to use cypress for my shelves..its very popular here. But using flooring is ingenious. I am using unfinished tongue and groove flooring for a kitchen work table, and staining them with the black tea and vinegar/steel wool solution.

https://www.pinterest.com/pin/12947917648661636/

use bricks in between boards.