A Tumbler uses a third party service to break the connection between a wallet address sending Bitcoin and an address receiving Bitcoin. So if you do not want anyone to know that you are using your Bitcoin wallet to purchase this book, then a Bitcoin Tumbler can protect your anonymity.
But isn’t Bitcoin Anonymous Already? While Bitcoin is more anonymous than any other form of transaction (eg. credit card), with a lot of work and ever-enhanced technology, Bitcoin block chains can be analysed and the Bitcoin traced to an individual wallet. (The block chain is public record of Bitcoin transactions in chronological order).
However, remember, if you are using a Bitcoin ATM, then there is no need to worry about tumbling. This extra security layer only applies if you have a Bitcoin wallet.