Book excerpt: Solvers who are ready for something hard will relish these tough crosswords. Every clue offers an extra touch of difficulty--a little twist or mind-bending pun that takes a bit of thought to figure out. Some are international, others historical, and still more feature some interesting, not commonly known trivia. Try to come up with the answers for these: "1636 Dutch acquisition," "Target of a bang-up job?" and "Spy exchanged for Gary Powers." It's a real brain workout. Answers: Aruba, Dent, and Abel.