The best way to do this is to create coupons for your book. If you go to your book's 'Settings' and select 'Coupons', you can click 'New Coupon' to get started. You can set various parameters for any coupons you create, including coupon price, maximum number of uses, start date and end date. Anyone you give a coupon code to can use it to buy the book from the book's purchase page. If you set the coupon price to zero, they can get the book for free, like a review copy.