As four months have passed since this question was posted, I am sure that the OP has by now found their way past this particular part of the puzzle game. However, in case others have this same question in the future, here's a more complete answer which builds upon that of @Echo...
The presence of the Mona Lisa, one of the most famous artworks by Leonardo da Vinci, is - as the OP deduced, and the hints suggest - a clue to look into the works of da Vinci. Polymath that he was, da Vinci was more than 'just' an artist - he was also a scientist, an engineer, an architect and much else besides. Much of his writings and design work in these areas is preserved today in collections of his manuscripts. Since da Vinci pre-dated the book as we know it today, these collections take the form of 'codices', comprising bound sheets of vellum (or similar materials) rather than paper.
Codices known to have been created by da Vinci can be found in private collections, libraries or museums all around the world. A list of the names by which these codices are commonly known can be found on Wikipedia. And this is where somebody attempting to solve this puzzle needs to turn their attention...
As @Echo found in their answer:
The nonsensical-looking text in the puzzle is a list of anagrams of the names of da Vinci's codices.
LAUNDER = Codex ARUNDEL
A CAT INSULT = Codex ATLANTICUS
FORBIDS FLIGHT = Codex on the FLIGHT OF BIRDS
ADD I MR = Codex MADRID
"SECRET ELI" = Codex LEICESTER
And the one @Echo didn't find:
A NUB SIR! = Codex URBINAS, more commonly known as "A Treatise on Painting" (Trattato della pittura)
With this in mind, we need to tackle the instruction 'Find book number five.' I doubt this refers to chronological order of the works, since it is difficult to be precise in dating these works, partly due to history and partly due to the fact that these codices are themselves relatively unstructured compilations of miscellaneous works written by da Vinci at various stages of his life and career.
As a result, I would propose that the answer you seek is simply the fifth entry in the list given above, which would be:
It is also worth noting that this theory may be backed up by a deleted answer to this question (link available only to users above a certain reputation level), in which a user claimed to have solved the puzzle by brute force and provided a spoiler that the answer 'starts with an "L"'...