To be the least intrusive I'd just ask for only the title of the book; anything more likely requires actually getting the book, plus filling out more form fields. If I'm reading Where the Wild Things Are to my kids I'll almost certainly know the title of the book (especially if I'm reading it several times). Other information requires good knowledge of the material (author) or pretty much requires the physical book (who remembers ISBN numbers offhand?).
Insert a little flavor into your form; "What are you reading tonight?" or "What books do your kids love?" are much more engaging than "If you could spare a moment of your time...". If you can make it sound a bit more personal; how personal depends on your audience, but sites for parents of children with disabilities tend to be fairly lighthearted.
Sending pictures is kind of a cute idea but may not get the participation rate you're hoping for. It might be an interesting way to engage people though; send pictures of your kid's book collection or something. (A possible problem would be eReaders and audiobooks; hard to take pictures of those). It would be a more complicated endeavor than an exit survey or similar though; I imagine it'd work much better with a closely knit community. I can see this working for a community library but maybe not the New York Public Library. Actually people are often fairly passionate about libraries in general, so it may not be as unfeasible as with some industries.
The thing about effective survey collection methods is that they put the burden of work on your staff, not the user. As with most things in UX, the more you can do for your users the better, because 10 hours of your work might save 10,000 users 10 minutes of their time, it ads up fast, and in this case it may well be the difference between completing a survey and abandoning it.
Collecting ISBNs would make the process automated at your end, but you'll get a lot less responses. Instead have someone ready some time to go through the supplied titles by hand (there may be typos) and locate that data on your own; only ask for what you can't find based on title. It's possible you may be unable to locate certain books, but throwing out a few extreme mispellings or obscure books from a large body of replies is better than a perfectly organized but much smaller body of replies.
