While the PDF is great for a quick search, It requires no battery, you can annotate it, and flipping through pages often helps you stumble upon concepts you didn't know you needed to review.

The guide covers the core pillars of mainframe development with a focus on practical application: COBOL Basics and Structure Overview | PDF - Scribd

The is a highly regarded resource among IT professionals, designed to provide a concise and comprehensive overview of core mainframe technologies. Often used for interview preparation and on-the-job skill updates, it covers essential subjects such as COBOL, JCL, VSAM, DB2, and CICS . Core Content of the Refresher

Mainframe Refresher by (often published by the MVSQuest Team ) is a highly regarded resource for both students and experienced professionals looking to brush up on their skills for technical interviews. Core Subjects Covered

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which covers COBOL history, TSO commands, and DB2. You can also find a Mainframes Refresher Part 2 for advanced topics. Academia.edu : Hosts the Mainframes Refresher Part - 1