Which of the following flight documents is essential but sometimes overlooked?

The aircraft's operation manual is indeed a critical document that is essential for safe and efficient flight operations. This manual contains vital information about the aircraft's systems, operational procedures, limitations, and performance data. It serves as a comprehensive guide for pilots and crew, providing instructions on various scenarios, including normal operations and emergency actions.

While the other documents listed have their own importance, they may not be as frequently referenced during flight operations as the operation manual. For instance, emergency contact numbers are important for communication during emergencies but do not directly influence flight operation procedures. Maintenance logs are crucial for ensuring the aircraft's safety and airworthiness, but they usually address the aircraft's past performance rather than current operational guidelines. Fuel consumption reports are useful for planning purposes but do not directly assist pilots in managing the flight itself.

In summary, the operation manual is often overlooked, but it is essential for providing the necessary guidance and information that pilots need to operate the aircraft safely, making it a document that must not be neglected.