[TallGuyShort]

He's written several excellent books on operating system design and they are considered by many to be excellent resources - he's taught a lot of people about how an OS works beyond the decision of microkernel vs. monolithic. He's a respected professor, seen as the authority on the topic, his less-popular opinion on microkernels notwithstanding. Most OS's today may have gone in the direction of being monolithic, it is true, but as other commenters have pointed out, Linux is actually very dynamic and modular, Minix 3 has some very advanced functionality, and this all validates the vision he had for microkernels, even if it didn't pan out to be commercially optimal.