As a former newspaper owner I had no problems with copyright. Many years ago it was common practice for journalists to change the opening par and then just make frequent references to the source throughout the article. Even though the story was basically in the same state as it was when 'lifted', there were no legal hassles.
Nowadays the internet has spawned a whole new concept of copyright...producing a horde of zealous writers who periodically make searches for any of their material online, and who then attempt to sue the owner/writer for copyright breaches.
The newspaper had little copyright trouble in this way because it was perceived as a large, professional entity. These days - when anyone can become a published author read by millions - the perception has changed dramatically.
We now have to think international, and to my knowledge there is no such thing as international copyright.
To be safe, I would rewrite all your material, that you have no source for, in your own words. This is the same method that researchers use to produce articles and material, claiming the knowledge as their own.
But with the sourceable material, it is always good relations to approach the author for permission, saying you'll attribute them and/or their publication. If done right, you'll not only get the permission given gladly, but make a contact and maybe a few more readers as well - as the author mentions it around. I do it - and it works!
Now - disclaimer time - I'm not a lawyer, so this advice must be taken with due care. It's based on what I see around me, not on any legal diligence. (Whew, I'm glad that part's over).
All the best,
*The Profitable Self-Publisher*