Would an offer of a free book of checks be considered a "bonus" under Reg DD if the value/cost is over $10?

No. I would use the following exception.

§230.2 Definitions.
Bonuses do not include value that consumers receive through the waiver or reduction of fees (even if the fees waived exceed $10) for banking-related services such as the following:

i. A safe deposit box rental fee for consumers who open a new account

ii. Fees for travelers checks for account holders

iii. Discounts on interest rates charged for loans at the institution