There are many ways to make a triangular bookcase, although most of them require that you play around with angles, mitre cuts or complex joinery. If you think outside the box, you can make an attractive and out-of-the-ordinary triangular bookcase by repurposing an old wooden ladder. You can widen the ladder legs to accommodate the size of your bookcase easily and you won't need a protractor or any fancy equipment at all.
Pull the legs of the ladder apart until you reach the desired "A" shape of your triangular bookcase.
Measure the distance between the legs at each pair of rungs, as well as the length of the rungs themselves. This will give the different shelf sizes for the bookcase. For example, if there are 40 inches between the legs at the bottom rungs -- measured outside to outside, so that the rungs are included -- and the rungs are 15 inches long, then the bottom shelf measurement will be 15 by 40 inches.
Cut out the shelves from 1-inch-thick MDF and place them across the rungs at their respective level. The shelves will get progressively shorter as they go up the ladder.
Attach the shelves to the ladder rungs with 1 5/8-inch nails hammered in every 3 to 4 inches from the top of the shelf into the rungs.
Paint the ladder and shelves in the desired colour, using a paintbrush. If you want a shabby-chic feel, distress the bookcase by sanding down patches of paint in random places.