If you'd rather get creative with your unwanted books instead of donating them to charity, this collection of used book creations should provide you with some inspiration.
I'm guessing most people don't have enough books laying around to build themselves book towers (or the space to do so), but if stacking is your thing, perhaps you can build some mini stacked book structures. Who needs blocks and Lego anyway?
I particularly love the beautiful lamp designs. How perfect would these be on your bedside table to illuminate the book you are currently reading?
Recycled Books as Art
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Dresses Made from Phonebooks
3-D Art Made of Books
Furniture Made of Books
Super Speed Commuter Trains