Lately, we have seen a lot of excellent guys releasing their valuable books for free! The time and effort they put into these must be crazy, and we thank them for it. I highly recommend you pick them up and give them a read!
vSphere HA Deep Dive 6.x:
This book was made available some time ago but still relevant, and all the thanks must go to Duncan Epping. You can download it here
Lots of thanks and praises must go to Cormac Hogan and Duncan Epping for making their Essential Virtual SAN (vSAN) book available for free. You can download it here
vSphere 6.5 Host Resources Deep Dive:
Lots of thanks must go to Frank Denneman and Niels Hagoort for writing this book which was a big hit at VMworld 2017. The book has been made available for free by Rubrik and VMUG, which you can download it here