If you refer to something as a taster of something greater, or of something that will come later, you mean that it gives you an idea what that thing is like, and often makes you interested in it or want more of it. The book is essentially a taster for those unfamiliar with the subject
One of a peculiar kind of zooids situated on the polyp-stem of certain Siphonophora
One who tastes; especially, one who first tastes food or drink to ascertain its quality