"This is the thickest book that I have ever read."
Translation:Ini buku tergemuk yang pernah saya baca.
I am not sure "tergemuk" (which means the fattest in English) is the best coice of words to use here. Perhaps "tertebal" is better.
Also, another version in Indonesian could be "Ini buku yang paling tebal yang pernah saya baca".