All 5 booklets in this set are very nice. All together they make a fairly complete rewrite of B/X rules for 5e sensibilities, and can be easily used to run whole campaigns. But book 4 is my favorite, because it's clearly a summary of a GM's experience and it successfully combines insights and techniques from both the older origins of the hobby and newer indie approaches - making players feel challenged while preserving their agency, using rules as a light framework that let complexity and fun emerge from the roleplaying instead of mechanical gimmicks, etc. It takes time to discuss some of the most common approaches in adventure building - like using dungeon world-style fronts for narrative-driven adventures, for example. In general, this book should be recommended reading for any GMs ready to lose the raining wheels.