Ambiguity usually presupposes some kind of preexisting vocabulary and syntax and in that case it's more appropriate to talk about word coverage, density, reading difficulty, and ambiguity tolerance. Some books are also challenging syntactically and it's helpful to distinguish between this issue and vocabulary load.
"Whereunto (in your opinion) doth this little flourish of a preamble tend? For so much as you, my good disciples, and some other jolly fools of ease and leisure, reading the pleasant titles of some books of our invention, as Gargantua, Pantagruel, Whippot (Fessepinte.), the Dignity of Codpieces, of Pease and Bacon with a Commentary, &c., are too ready to judge that there is nothing in them but jests, mockeries, lascivious discourse, and recreative lies; because the outside (which is the title) is usually, without any farther inquiry, entertained with scoffing and derision. But truly it is very unbeseeming to make so slight account of the works of men, seeing yourselves avouch that it is not the habit makes the monk, many being monasterially accoutred, who inwardly are nothing less than monachal, and that there are of those that wear Spanish capes, who have but little of the valour of Spaniards in them. Therefore is it, that you must open the book, and seriously consider of the matter treated in it. Then shall you find that it containeth things of far higher value than the box did promise; that is to say, that the subject thereof is not so foolish as by the title at the first sight it would appear to be."
Gargantua and Pantagruel
Unique words: approx 26,000
High vocabulary density
This is one of the most vocabulary dense books in the Project Gutenberg database. It's also quite a tease if you're into French classics.
French audiobook:
http://www.litteratureaudio.com/livre-a ... gruel.htmlhttp://www.litteratureaudio.com/livre-a ... antua.htmlEnglish translation
https://www.gutenberg.org/files/23033/23033-0.txtMoll Flanders (an old "classic")
Unique words:approx. 6100
Low vocab density. ("easy")
You can take a look at the "choosing extensive reading materials" thread for some ideas and classifications of books according to relative difficulty.