"What If? 10th Anniversary Edition" by Randall Munroe. It's the updated version of the original "What If?"
The text is mostly the same as the original, with matching page numbers up to page 295, but with lots of new content inserted in red annotations along the side. Those new content include explorations on what happens when you multiply some effects by 10x (as promised on the cover) and various clarifications, and also some new jokes and illustrations, plus a bonus chapter starting on page 296. Particularly noteworthy is how the "Common Cold" chapter got updated with actual observations from COVID-19 pandemic. Also, the two typographical errors from the original edition are both fixed.
In short, this 10th anniversary edition is a superset of the original, and has enough new content to justify its worth. Definitely get a copy even if you already have the original book.
Previous (2024-12-09): Standing figures
Next (2025-02-09): "Echo: The Oracle's Scroll" by bumbleborn