This is not a DIY project. This is me saying that instead of paying for a cheap, flimsy bookshelf for all your textbooks, you could consider making your books into a shelf. I work in a bookshop and have perfected the technique of book-stacking. Its very simple: put the biggest books on the bottom and pile them up in order of size so the smallest are on the top.
If you’re feeling adventurous, why not try making an actual bookshelf out of your books?