If you get a free book as "thank you" for allowing to use the pic, I'd say it is worth saying yes (except if you value your picture as more than that)

I agree with Sandie. It's an honourable appreciation of your work/snapshot, you can help someone else, an without any costs and effort you get a tiny bit of prominence.

My opinion is that you should go for it. I would find it nice to see something of mine in print (it obviously means that you either found an interesting subject or took a particularly good shot) and the guy has been good enough to ask. These days, so many people use pictures without consent even though asking is simple good manners.

I had someone message me on Flickr about using one of my photos in a book on Ferraris. "If you grant me the possibility to cite your references (company name, address, website, email…) in the "Photo credits" space, I would be pleased to send you a copy of my book.
The publication of the book is planned at the end of the year on 2016, at the beginning of year 2017."

Should I say yes?

It often happened with IMCDb pics (obviously :D)

For my own pics, I remember that long time ago I saw that someone re-used the a photo of my PC (that I posted on a forum) to sell an identical PC case on Ebay. He probably just took one of the first Google Image search results.

There were probably other cases, but none that I remember for the moment

Did some guys ever got the surprise to see some of their pics being used on other websites that the one they post them on ?
Here how the story starts : i was looking at some old topics here in the forum and came across the " Explain your username " one ( not really the title , i don't remember, so i just put in few words the idea of the topic ). I was tempted to answer to it ( even if it doesn't have an interesting story behind it ) when i realized that if i was to explain Baube, maybe i would have to do same with my Flickr one, Zytonits , since its a different one ( Baube was taken when i register in Flick )
So i decided to check if Zytonits had a real meaning in some language and when i googled it, there was a result about Nissan Rogue being recalled with a image courtesy of Zytonits in the description . Curious, i went to see and got the surprise of seeing the pic i took at the 2015 Montreal Auto Show. So i tried to see if other pics were used and found
The Nissan Rogue that triggered the search
Ford T-Bird Turbo in the 6th place
Nissan Titan
Ford Explorer Sport Trac from same website
Mazda CX-3 which i did not even realized it was RHD before seeing the pic on that website
They also used a Honda Fit and the Subaru Legacy Concept but can't find them right now due to computerosaurus struggling when too many things are open
Acura ILX getting more popular, being found here and here.
Even the non car-related pics found their way somewhere else....

I don't mind that my pics are used as i did free them but just really surprised that somebody could be interested in my ( very, very amateur ) pics

About the CX-3, if we have a japanese speaking member, would be nice to have the big lines of what they are saying.