First of all, each of these (mostly made-up) book titles either clues or actually names both a country and a number.
Moreover, although the books are (mostly) made-up, each one can be assigned unambiguously to a two-letter subclass of the Library of Congress Classification. For example, The Holy Trinity in Khmer Catholicism belongs to subclass BR, for Christianity.
As it happens, each book's LCC subclass is also the ISO two-letter country code for the country mentioned by another one of the books in the puzzle. (I know, right? What are the odds?) In fact, the books can be paired up in this way, the country code for one being the LCC subclass of the other and vice versa.
So what you do is, you take the number that appears in each title, and use it as an index into the title that title is paired with. So The Holy Trinity in Khmer Catholicism clues 3 and Cambodia (KH), in subclass BR; its counterpart is Lex Brasiliensis, volume VII, cluing Brazil (BR) in subclass KH, and so you extract the third letter of Lex Brasiliensis, volume VII, which is X. Meanwhile, Lex Brasiliensis, volume VII indicates 7, so you extract the seventh letter of The Holy Trinity in Khmer Catholicism, which is Y.
Sort the extracted letters in order by the LCC subclasses of the books that produced the numbers for extraction. In this order, the letters spell INDEXES PATHOLOGY DICTIONARIES. The LCC subclasses for Indexes, Pathology, and Dictionaries are respectively AI, RB, and AG, so the answer to the puzzle is AIRBAG.
This pdf summarizes all of the data associated with each of the titles in this puzzle.