1. Yes, you can take the upgrade exam. Lookup the prerequisites for the upgrade exam: it says you have to be SCJP certified for any older version. So if you have an SCJP 1.4 certificate, that's fine.
2. Why would you do the full exam? There is no difference between the certificates of the upgrade and the full exam: there is just one SCJP 6 certificate. It will not say that you did the upgrade exam. The upgrade exam has fewer questions (48) than the full exam (60).
4. There are a number of good books; the SCJP Study Guide, also known as "K&B" (the first letters of the names of the authors), is one of the best and most popular. There are no books especially for the upgrade exam, since the material covered is the same as the full exam.