I'm on a Mac and in order to create a working link I first click the link icon on the upper right  - it looks like a little globe with a chain in front.  A window should open.  Cut and paste the URL into the box.  That should do it!

You can do it a couple of ways. The easiest is to copy the PBS link and paste it into the post. For some reason lately, its been copying text boxes on me if I copy the link from a list so I have started to copy and paste the link from the yellow popup window you get when you hover over the cover image.

The copy/paste book links seems to work differently in FF v IE sometimes so I am not sure if that method will work for you.

The other method to use is the link button in the options on the post reply box. What I do it type out the name of the book, highlight that, then open another PBS window and go to the book page, copy that URL, go back to my post, hit the link funtion (little world with a chain link on the right end of the menu) paste the URL in the window that pops up.

There is a tool bar with the link at the top of the text box.  The toolbar can be opened or closed with the little triangle.  I know the first time I tried to link with the new system, my toolbar was closed and I was totally confused.

Several methods are discussed in this discussion thread.