That said, the humble PDF remains the most universal, DRM-free (if purchased correctly), and permanent format. A PDF you buy today will work on a device ten years from now; a proprietary app subscription may not.
: While technically a comprehensive textbook, its detailed anatomical plates and procedural diagrams are so extensive that it often serves as a primary atlas for complex lung resections and esophageal surgery. Pearson's Thoracic and Esophageal Surgery thoracic surgery atlas pdf