I've sold quite a few photos over the years - a lot have been to smaller companies like you're describing.
If they don't want to provide attribution - first, you need to make sure you have documentation and signatures from the firm's contact to protect your rights as the original photographer. Second, because your work is being distributed without being acknowledged as yours, you are losing out on possible leads to subsequent photo work (unless the firm replies to inquiries and steers people your way). After all, if your name isn't listed on their materials as the photographer, people will probably assume that someone in the firm created/provided the photo so for me, I'd want to recoup those lost leads via the licensing fee.
With that, I think at minimum you should ask $100USD but depending on how unique or customized the image is for this firm's needs - I might charge substantially more. If you're willing to share the image here along with more details, I'd be more than happy to offer more advice. mSeattle, you're more than welcome to pick my brain and save the $ on the book