I'm not an expert on ISSUU, but I'm more than happy to share what I have figured out.
There are thousands of publications available for anyone with a web connection to view for free on ISSUU. You don't have to have an ISSUU account, but accounts are free and enable you to save related publications in a "stack" so you will be able to find them easily in the future. It can be a little hard to find publications you are looking for since things are posted worldwide and might not have the keywords you expect. It's nice to be able to save them in a stack. You can have multiple stacks, too.
I have found searching on pattern drafting, sewing, needlework, etc to be fruitful.
There is a teeny bit of a social media aspect to it because you can follow publishers and you can follow another user's stack. For example, I follow burda style, and I follow The McCall Pattern Company and my own Pattern Drafting stack has 4 followers. I don't know who they are, I just know the number of followers.
I generally see these types of publications on ISSUU:
Magazine previews posted by the publisher in order to entice readers to buy the full magazine.
Magazines posted by the publisher, excluding the "extras" meant to entice readers to buy the full magazine with the extras.
Scans of magazines posted by someone other than who I know to be the publisher.
Scans of magazines that are now in the public domain posted by any interested party that wants to scan and publish them.
Current book previews posted by the publisher in order to entice you to buy the book.
Scans of currently in-print books posted by someone other than who I know to be the actual publisher.
Scans of out-of-print versions of currently in-print books by someone other than who I know to be the actual publisher.
Scans of books that are now in the public domain posted by any interested party that wants to scan and publish them.
Catalogs published by retailers.
Graduate student's self-published work.
Professional's self-published portfolios.
I'm not very comfortable when I see currently in-print magazines and currently in-print books that have been posted by someone who is not obviously the rightful publisher. Likewise, with out-of-print versions of currently in-print books. I choose to use these only as a tool to help me decide whether to purchase the book/magazine or not, rather than reading these cover-to-cover. In my mind, it's kind of like a flip-through at the book store.
If others have concerns with copyright, you can see the publisher's name in ISSUU and decide for yourself. I believe that there is a process for authors and content owners to follow with ISSUU if there is any misuse of their work, as well.
I hope that helps!