Thomas Keightley - Secret Societies of the Middle Ages (1837)
Archive.org direct link
Google Books direct link (expanded edition)
This had some recent interest, resulting in reprints from Dover and Inner Traditions among others, though despite the connotations of the title today this is more genuine history than conspiracy speculations. As you could guess from the title it focuses on the Assassins, Templars and Westphalia tribunals. I've heard it was the first English-language book with an extensive treatment of the actual Assassins.
Thomas Keightley (1789-1872) was a prolific Irish writer who mainly focused on mythology and history. This book and 1828's The Fairy Mythology are his best remembered but as with so many who scribbled for cash he covered a wide range - the war of Greek independence, the Crusaders, several works on Milton, an edition of Shakespeare, editions of Horace, Virgil and Ovid, translation of a Dutch novel, ancient Rome, numerous textbooks and educational aids. The DNB claims "But he ludicrously overestimated all his performances, and his claim to have written the best history of Rome in any language, or to be the first to justly value Virgil and Sallust, could not be admitted by his friends."
There are two download choices. For some reason this isn't listed in Open Library but Archive.org has a scan (actually two but this is the better). Google has a more clean scan of an expanded edition (apparently more info on the Assassins) but as usual they hide the download choices. Click on the small options wheel in the top right and there are PDF and epub selections. (The link in the top left will put it in your Google Play account.)