This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
With respect to HashSet and LinkedHashSet, you basically need to understand that these are Sets (no duplicates); and the LinkedHashSet maintains element insertion order, while the HashSet maintains its own internal ordering. (A TreeSet maintains "natural ordering" of elements.)