I'LL BE THERE by Iris Rainer Dart (Little, Brown: $19.95; 253 pp.). If you haven't read "Beaches," this sequel won't work very well for you. But if you have, and if you enjoyed it, you'll like this one as well--although, as is the way with many sequels, it's a lesser book. These are lightweight stories anyway, bedtime or beach-blanket reading, full of flaky Hollywood types, punchy one-liners and pop-psych emotions. The heroine is Cee Cee, egomaniacal superstar with the vocabulary of a truck driver and a heart of gold. When all is said and done, this is a story about a kid growing up in Movieland. There are some amusing moments and a few tugs at the heartstrings; the stuff of which films are made.