Complex sentence: The acquaintance to Ramsay had led the Foulis brothers to the most fine and exclusive Parisian libraries, of which a good number of the contents had become the Foulis brothers' initial success that had launched their book dealing business.
Split into simpler sentences: The acquaintance to Ramsay had lead the Foulis brothers to the most fine and exclusive Parisian libraries. Libraries of which a good number of the contents had become the Foulis brother's initial success that had launched their book dealing business.