What's the best source for practice cases? Are casebooks the best to practice with?
the first book I would practice with is Cosentino's. Then, I strongly recommend you to work with real cases handled by your friends/mates/coaches during their interviews: these are the most similar problems you are going to face with.
Other great materials I find very useful are MIT Sloan casebook and Preplounge' (https://www.preplounge.com/en/management-consulting-cases).
Only as a very beginner. The earlier you start practicing with experienced candidates who have the cases from the real interviews and can give you good feedback, the better
When I practiced for the case interviews as a total beginner, I started with Victor Cheng's Case Interview Secrets book to lay out the foundations. You may find this useful.
Going forward, the best way to practice is to practice with others. There are multiple casebooks that I find quite useful, you can find the link below to access them:
here the strategy I followed to prepare for McKinsey:
1. Read "Case in point" of Cosentino
2. Prepare a sheet with all the most common framework you may need to solve a case
3. Download various cases and practice by doing cases by yourself
4. Download audio of Victor Cheng and understand where the interviewee makes mistakes in order for you not to make the same mistakes
5. Practice as much as you can with someone else by using preplounge
Hope it helps! :)