Can an insurance carrier pay 'book value' for a truck?

WHOSE book? WHICH value in the book?

The answer is, the policy will pay whichever valuation you purchased, when you bought the policy. The most common valuation is "actual cash value". Look, money you put into it to keep it working, does NOT increase the value of the truck. It doesn't! It makes is a working truck. Anything that passes inspection and runs, is worth at least $1000, as long as it doesn't have a salvage value. If you dropped a $5000 motor into a $500 non-running truck, that does NOT increase the value to $5500! It only makes it a running truck, that passes inspection.

If it has a salvage value, expect HALF the "book" value - book being, nada, edmonds, or blue book PRIVATE SALE value, not dealer retail

Going to small claims court isn't going to change anything, he's still only legally entitled to (from the at fault party) the actual cash value of the truck - not gas, oil changes, maintenance, repairs, etc. NONE of that counts. And, unless he bought an "agreed amount" policy - highly unlikely, on an old work truck - all he's going to get is actual cash value - minus his deductible if he claims on his own policy.

That's how insurance works. They pay fair market value only

Going to small claims court would not work for two reasons

1) The insurance company is correct

2) Even if the insurance company was wrong you would still need reciepts

If the truck is ancient and not worth much, it is in the insurance company's best interest to pay book value and be done with it.

They can, and they must. They have no other options. They cannot pay more. The only two things that you can have them do are to pay only the amount that the law allows them to pay, or to have them pay nothing. Even if he goes to small claims court, they still cannot pay him more than the law allows. They cannot pay more than the value, and they certainly cannot pay enough for a new vehicle. Paying either of those amounts is against the law.

The insurance company will only pay him "fair market value" for the truck, NOT what he put into it. Small claims court will do nothing for him.