If you're using the Post-9/11 GI Bill, you are eligible for a stipend for books and supplies. The stipend is capped at $1,000 per year, but your payment amount actually depends on how many units or credits you're enrolled in. The payment and the cap are also pro-rated according to your eligibility tier.
Here's the formula: $41.67 x number of credits x your eligibility tier = book stipend, not to exceed $1,000 per academic year.