If you need the cheapest bookshelf ever, for a big amount of books, try this one out.

This may look like any other bookshelf, but there are a few things about it that makes it superspecial.

The design is reduced to a single symmetrical element which is used for vertical and horizontal segments. The dimensions are the results of the material size, without any leftovers.
Each element is around 15cm wide with the 5cm deep slots, every 30cm. When connected the depth of the finished bookshelf is around 20cm. Vertical segments are set against the wall, and the horizontals are detached thus allowing the air flow behind the books.
The number and distance between vertical segments allow the material to be thinner = cheaper, retaining the strenght.
It is made with only one tool - circular saw.
From only 2 OSB (Oriented Strand Board) 250x125cm just 10mm thick, you get a 2,5x2,5m bookshelf.
It is cheaper than BILLY bookcase from IKEA which you need 3 pcs to get the same bookcase size (3x70=210$), OSB 2x12,5=25$

It is possible to change the dimensions depending on your space, it can be wider, it can be higher, it can follow your sloped roof...

Let's start:

Step 1: Tools and Materials You Need

The only real tool you need is the circular saw, but here is the complete list.

Tools you need:

pencil
steel square
measuring tape
circular saw
stands
chisel
clamps

Materials you nead:

for the bookshelf 2,5m x 2,5m

OSB (oriented strand board) 2pcs (1cm x 125cm x 250cm // 7/16 in. x 48 in. x 8ft.)
1 in. Zinc-Plated Corner Brace 4pcs

for the jig

a piece of wood beam (5cm x 7cm x 30cm // 2in. x 4in. x 1ft.)
a piece of some thin material (3mm x 5cm x 35cm // 1/8 in. x 2in. x 1ft.)
a few nails or screws

Step 2: Cutting Into Strips

Fasten together both OSB with clamps, so that you have to measure and cut only once.

Divide and than cut each board onto 8 equal strips to get 16 pcs. The width of the strips depends on the thickness of your saw blade and the width of your board (in this case 153mm).

Step 3: Cutting the Slots

Fasten all the 16 pcs of OSB strips together with clamps.

Now you need to make 10mm wide slots, 5cm deep (which is the depth of the saw blade).

Make 2 marks 7mm apart, every 303mm, throughout the strips.

To make it easier and more accurate make a simple jig from the wood beam and the other piece of thin wood as showed on the picture.

Cut the slots in 3 passes, and clean the bottoms of the slots with chisel if needed.

Step 4: Assemble the Bookshelf

You need 2 persons to assemble the bookshelf. Start by leaning all the vertical segments on the desired wall approximatelly on their place. One person holds 2 verticals upright and the other joins the highest and lowest horizontals. After that you connect all the verticals, and in the end the horizontals.

Fix th ebookshelf to the wall with 4 1in. Steel Zinc-Plated Corner Brace.