Audit working papers

Audit documentation:

The purpose of audit documentation are to provide: a sufficient and appropriate record of the basis for the auditor’s report evidence that the audit was planned and performed in accordance with ISAs, and applicable legal and regulatory requirements.

Working papers are part of audit documentation, but are usually described as something like:

The documents which record all audit evidence obtained during financial statements auditing, internal management auditing, information systems auditing, and investigations.

So, a letter of engagement is part of audit documentation, but not really a working paper. You would need to concentrate on saying that, for example, a WP lists the precise evidence seen, conclusions of audit procedures, review points etc.