TaiHT,
I commend you for being very courage to be upfront about this. Just what others have said these issues with thoughtful perspective, and some in facts about copyrighted materials.
Now, when you mentioned photos shared on the Internet. I have to say 'be careful." Because most of time, we don't know the original owner of such photo on the Internet. Majority of Internet users usually pass off other photos on the net, then other people on the net continue to pass it off, so on. Viral like cancer. That is one thing about Internet.
As for trying to contact original owner of photo, I wish you lots of success in obtaining the permission and such. For some, be prepared to pay for some photos. As for 'unknown' source of original owner of such photo - it is because it is probably pass off on the Internet.
It is ALWAYS a good idea to look into EACH photo's metadata/EXIF data that is built inside the image file. It should include copyright info, name of photographer or original owner of this/that/the other photo, so on.
In summary, if I were you, I'd make my own eBook with any photos that is VERY original and I *OWN* all of photos - which means I take photos with my own camera. Such original photos constructed and assembled in your own book tells interesting story.
I bid you lots of luck and good luck in securing/contacting/obtaining the permission for EACH photos. Also for any contents copyrighted by such author(s) and group of authors if any. It is time consuming task, if you want to do this venue, then you *MUST* do this. Otherwise, you could get into trouble later on.
In closing, all I can say proceed with caution and follow through all source cited, obtaining ALL permissions (and pay for photos). Not all photos ARE free, just in fact.
Best of luck!
P.S. May I add few things. For each permission obtained FOR EACH photo(s), it is wise for you to have a long list of more detailed infomation on endnote or acknowledgement page or something like that.
I agree with Dave's word. You cannot put copyright info on your book that you don't own all of these photos/content. Extremely cautious!!! If you are not careful, this means legal problem later on.
Additional note:
I have found that, for example, I had posted some of my photos on Facebook (even though I don't post many photos on Facebook). I put all of EXIF metadata with copyright info and everything. After I uploaded few photos on Facebook, I was astounded to learn that Facebook deliberately remove all of EXIF metadata. So, don't be surprised that you probably don't see much of EXIF data on many social media websites. There are well-respected websites such as Getty Images, as an example, Getty Images is among one of respected stockimage agency where it keep all of EXIF metadata in each photo files. Therefore, it is a good idea for you to open image in Lightroom or Aperture app to look into EXIF metadata record. If there isn't any, be doubtful. And be careful!