Hi Mike,
Please don't give your work away. Insist on a reasonable fee for usage of your image, and make sure you will still retain copyright ownership. Publishers will often approach photographers who they consider to be part-timers or hobbyists with the lure of "photo credit" so they don't have to fuss with us working Pros and our need to make a living and control usage of our images. Please show them that you cannot be bought by a meaningless mention of your name in 6 point type.
Book usage example according to Fotoquote Pro: 1/4 page, 20k to 40k press run. Usage fee should be between $191.00 and $393.00 US. Higher press run and or page spaceprice goes up.
Thanks,
Mark