Get an old VHS tape at a thrift store -- should cost less than a dollar.
Be sure to select the kind with a snap-closed clamshell case, not those that just slide the cassette into a sleeve.
Pull out the tape and discard it (or watch it, if you have a VHS machine and found a title you like).
Put the book where the tape was, close the case, and your book is protected.
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Get hold of some of the cardboard folders/envelopes/mailers that Amazon and similar companies send books out in.
These are specifically designed to keep books from damage.
It will add a little to the weight of your backpack but so would almost any other solution.
If you don't have any yourself, ask around.
A lot of people keep them for recycling or re-use.
Bibliophiles tend to build up a surplus and would probably be glad to let you have a few in various sizes.
Alternatively, get an electronic reader!
In my experience it pays for itself quite quickly and it certainly allows you to carry many books at the weight of one.
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I have used cereal or granola bar boxes and cut the ends off of them.
Many of the larger snack boxes are bigger and you can fit a few books in there.
They don't last a terribly long time, as they themselves get banged up (instead of your books!)
but they are cheap and easy to replace!
You can tape them with duct tape for higher durability.
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Put the books in a big plastic cover or kit.
Put this cover/kit in your backpack.
This will not add much weight to your backpack.
lifehacks.stackexchange.com/users/10930/MANEESH MOHAN
The simplest thing to do would be putting the book in the bag with the spine facing down and nothing above it between two solid items.
Beyond that, you can use something like a lunch box or Tupperware.
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I would take a few lengths of fabric (sewing) elastic and sew or glue them in a grid fashion to the side of the backpack that is along your back.
The vertical strips should end a fa inches above the base of the backpack and all the strips should be far enough apart to allow the book to slide in with some stretching.
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If you carry a laptop or tablet: buy a protection sleeve for the tablet and slide the book in it as well; usually, the sleeve leaves enough room to squeeze in a book without leaving wiggle room.
Maybe you could also wrap it in a piece of cloth and secure it with a safety pin?
Also, there are dedicated book bags which may offer some protection, such as this random example from Amazon.
Stationary shops might offer plastic foil to reinforce the cover of the book (another random example from Amazon).
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Cover your books with a makeshift newspaper cover or clear plastic wrap which is available for especially covering books
O r Protect the spines and corners by taping the corners with thick tape like them electrical ones
lifehacks.stackexchange.com/users/25619/ARIA
I always like carrying books around but at the same time I hate getting the corners of the books bent over time and I try to keep them as perfect as possible.  Given that most of my books are paperback editions and that I usually carry them around in a backpack, how can I transport them without bending the corners and damaging them?  I've tried putting some kind of paper cover around the book but I'd like to know if there is any really effective way of doing it.
How can I transport a book in a backpack without getting the corners bent?
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lifehacks.stackexchange.com/questions/10156
lifehacks.stackexchange.com/users/10881/A. A.
There are a few different ways of protecting books when carrying them in a backpack. One way is simply to carry them in a protective case, which could be an old VHS case, a lunchbox, a cereal box or a big plastic cover. Another method would be to place the book with the spine facing down and two items either side with nothing above it. Alternatively, you could wrap the book in some cloth. The options are to use an old VHS tape box, a cardboard envelope used by online retailers, a cereal box, lunch box or large plastic cover, strips of elastic sewn into the bag, a laptop protection sleeve or a clear plastic book cover.
Use an old VHS tape box. Use one of the cardboard envelopes which online retailers send books in. Use cereal or granola bar boxes with the ends cut off. Use a large plastic cover. Use a lunchbox or put the book in the bag spine down with solid objects either side. Sew or glue lengths of fabric elastic into the bag to hold the book. Use a laptop protection sleeve. Use a newspaper cover or the clear plastic covers made specifically for books.