Read this order
Book → Letter: Information becoming available in writing: study notes, records or a disclosure in a document.
What changes in reverse?
Letter → Book: A confidential document or a message with details that are still unknown. It can also describe written study material.
Letter → Book ↗A practical example
Study notes becoming available give Book + Letter a simple example. A document with details not yet disclosed fits Letter + Book. The question determines whether learning, confidentiality or missing information is the useful emphasis.
Check it against your question
Keep the actual order and ask which part of the example fits your situation. An example illustrates a possible reading; it does not establish facts or promise an outcome.
The meanings behind the pair
Book: Book represents knowledge, study, privacy and secrets or information not yet disclosed. It may be a literal book, education or a matter that is still unknown.
Letter: Letter is a written message, document, notice or record. It draws attention to the information itself and to how it has been communicated.
These examples show possible ways to read the pair. Your question and the other cards may change the meaning.