The book appears to be incorrect. Changing a database from read/write to read only will not make the transaction log disapear.
The only way to have a database which doesn’t have a transaction log is to use a database snapshot as the snapshot doesn’t have a transaction log.
I ‘m also working through the MS Press book and found your question here because I had exactly the same issue whilst working through the Suggested Practices. I’ve found quite a few mistakes in this book and since I’m looking to undertake the exam I’m rather annoyed by Microsofts poor quality control when producing this book. I would have thought they’d have someone work through the exercises to ensure that these sort of errors are corrected before the book is published. It seems they’re adopting the same attitude to their training materials as they do to their software. Release it with bugs and let the customer find and report the error. Doesn’t inspire confidence does it!