You're right, at this stage you need to find out a few details. eg exactly what the photo is being used for, where it features in the guide (front cover?), how large it will be, how many copies are being printed, where it's being distributed, how long it will be available for etc etc.
The main thing to remember is that your work has a value and that just an image credit if effectively worthless. In fact if anything it de-values your work and the industry in general.
The fact that they've approached you asking to be able to license it is a pretty good sign I reckon. I usually get something along the lines of 'we really like your photo and would like to use it but we don't have any budget but we'll give you a credit for it'. Always pretty tempting to ask them if the printing, design work, distribution etc is also all being done for free in exchange for a credit?? No, I thought not so why should photography be any different.
It does feel strange to put a price on your work but it's worth it
