If you have something to say then you should add it.

If it's a comment then comment and if it's a new bite of information then add it. Sometimes you may be able to write a "better" or fuller page based on the page of another user. That's fine, although good form suggests an attribution is called for.

Over time the book may get heavy and we may need to remove less usefull pages (maybe) but at this stage the book is light and empty so add your stuff.

Links

There is no reason why you can't add links and quotes from your own content (blog, forum, wiki, etc) so long as it has some relivance to the content.

In this way a blog author or webmaster might be able to promote thier own work while adding to ours.

A win-win set-up.