Add a supplemental recommended reading list.

Since the scope of the project is only to get junior admins up to speed,
a supplemental reading list of excellent and well-written books seemed
appropriate. This list is only a starting point, as there's plenty of great books out there I have not read personally. :)

What do you folks think of the idea?

As for the table, I don't like RST grid tables because of how much work they are, but it seems the best way to present the information. Totally not tied to it, and would welcome a better approach if there is one.

I haven't added a new sidebar link before, so I'm not sure if I did it right.