I read The Hippo System by Eric Briffoz and Gary Gifford and I felt the book was just okay as a supplement for a more detailed Hippo book. The Hippopotamus Defence by Alessio De Santis was better and more detailed. I have not checked lately for any other or newer Hippo chess books so I am not sure at this time what the best Hippo System chess book would be. You could research some on YouTube as every now and then a random chess master produces a great video on the Hippo.
(Play the Hippo Attack and Defense and the The Hippo System) Books by Eric Briffoz
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Any thought on his books