"This is the thickest book that I have ever read."
Translation:Ini buku tergemuk yang pernah saya baca.
I got answers from native speakers. All of them argue that "gemuk" should not be used to describe a book even as a metaphor.
"Gemuk" is used only for human beings (or animals). The only exception for non-animals is "fertile soil or plants" -- This usage is registered on KBBI. Please check it out!
I also wondered whether the original Indonesian sentence could be interpreted as "this book contains high quality of context and very helpful for my education" (i.e. gemuk as "rich context"). In this case, the appearance of this book is not necessarily thick like with 800 pages. But one of the native speakers taught me that "Berbobot" (have weight or quality/values) could be used for such an interpretation, but "gemuk" is never ever used.