If a reader already has a book, it's advisable to read that instead of a summary. Readers who are considering purchasing a book may find a summary helpful in deciding whether or not to buy it.
The main drawback to reading a summary before a book is the possibility that the summary spoils the book for the reader, either because he knows what happens or feels the summary explained everything in the book adequately already. However, the summary can help if the reader isn't sure whether he wants to devote time towards reading the entire book.
While some readers use summaries to decide on purchasing a book, it's more common for readers to use summaries when they want to know what happens in a book without committing time towards reading it in its entirety. Reasons for this include the book being required reading material for a class, or a desire to know what happens in a popular book.