I reward myself with new books although I have a huge stack awaiting reading, visiting in person with local friends, extra time on Facebook, and a great meal! My beautiful pom Hallie gets a usually forbidden treat!
I read trilogies both ways! If the interval between releases is fairly long, I wait, but, if they are released 1 - 3 months apart, I read them as I get them. Sometimes it all depends on the author as well; some authors I just am too anxious to read!