Is it possible to enter time entries for projects synced from Zoho Projects to Zoho Books directly in Zoho Books?

No, time entries for projects synced from Zoho Projects to Zoho Books cannot be logged directly in Zoho Books. Zoho Projects remains the primary application for managing time entries and project-related activities. Any time entries should be logged in Zoho Projects, and they will automatically get synced to Zoho Books through the integration.