Sure! I would highly recommend that if they have an Amazon account, that they order for themselves (they might want to back the project at another level to get more rewards!). But you can certainly buy more for them through your account.
For US orders, please add $45 to your pledge for each additional book set you want to purchase.
For Canada orders, please add $50 to your pledge for each additional book set you want to purchase (I subsidize shipping to Canada, but with more books/heavier packages it starts to cost me a lot!). So if you want to buy a physical book and all DLC for yourself at the $70 level, plus two more books for friends: $170.
For international orders, please add $60 per additional book. I priced it out; I'm still subsidizing your shipping price a huge amount (shipping is incredibly pricey)!
On top of that, you can upgrade your additional copies to the limited edition hardbacks by adding $15 extra per copy.