I have a different opinion on this.
I understand that providing direct links to books can be problematic, as suggested in this answer.
What about comments like this. (The first one is similar to some comments I've seen here - but I've reworded it substantially, I hope the rewording was sufficient to make difficult to find the original comment and the identity of the poster of this comment. This comment is more-or-less in the spirit of J.M.'s comment above. The second one is basically the same as a comment I've posted a while ago and then deleted after I found this thread. I've changed this only to a book that is legally available online.)
- This book might be downloadable in the electronic form from the usual places.
- It seems that some copies could be found in the Internet, as Google search for
"graph theory" diestel djvu or
"graph theory" diestel torrent suggests.
But there is no doubts that these copies are not legal. (Also you should be careful when using the sites like this, because of viruses and stuff.)
I am far from being legal expert, but I have hard time imagining that posting a link to results of Google search might be illegal. So I think the only problem is whether MSE community would object to something like this on moral grounds.
Of course, I will follow community consensus on this issue. (As the moment when I am posting my answer, Willie's answer has 19 upvotes so it seems to be the general opinion. I wanted to provide an alternative viewpoint.)