Complete works of Shakespeare, I guess? For an ebook though, I have an 11k-page single file that has all 15 Wheel of Time books, that was a fun read, June through November
I have a dictionary of English's most unpronouncable words (90% of it is simply medical or other scientific words). It's like 2 feet thick and as big as a coffee table.
I only bought it because of its absurd size and it is also the most expensive book I own (IIRC it was about $200).
The thickest one is probably "The joy of cooking" -a cook book of course. If not that than a "Sword of truth" series book cause those are pretty thick too. Maybe I'll grab a ruler when I get home....and some bugles of course.
I have a 词海 hanging around. That's a single-volume dictionary of every word in the (modern) Chinese language. It doesn't qualify to be a pocket book for certain and not even a handbook. It belongs on a podium like a medieval tome.