4 coats of Armour Seal. Or replace with ash, oak or walnut

 

bookshelf ??
there really isn't a lot of wear and tear on bookshelf boards. (compared to a coffee table).
I am thinking that any good quality polyurethane will work just fine.
depending on your skill level for application methods and products you have access to..
if you over think it ~ it will surely be over thunk !!!
best of luck in your project.
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Not sure why you need to harden a bookshelf, but if you really want to, you can always use epoxy. Of course, it sort of defeats the purpose of using pine, which is the cost.

 

Short of infusion, nothing. About the best for the hobbyist is poly. Thin regular ob poly, like minwax, 1:1 with ms, flood on the surface for 10 min, keep it wet, then wipe off, repeat after it dries. This allows the finish to soak in as far as its going to. Add coats with the desired method and film thickness. The wood fibers below the finish are still soft and will fail creating a dent if something hits it hard enough. Arm r seal is 1/2 the solids of mw, so no need to thin it.