When I think of secret history books I think of books that fall into the following categories


Romance of the Archive: A book which depicts an acquaintance with moldering books, indices, monographs, bibliographies, slips of paper, and footnotes through a romantic lens depicting a scholar-adventurer whose quests for Truth take place mostly through archives and dealing with written matter. A pivotal work is A.S. Byatt’s Possession.

Fantasy of History: A story that uncovers a Secret History of the World through the aid of fantasy devices. Set in the mundane world, either now or at some point in the past, no matter how fundamentally our understanding of the mundane world may be rewriten through the revelations of the tale. These are also widely called Secret Histories.

Alternate History: Simply stated, an alternate history is the description and/or discussion of an historical "what if" with some speculation about the consequences of a different result.

Occult Fantasy: The study of the occult is the pursuit of hidden or secret doctrine. Occult Fantasies are stories which explore these mysteries in the hope of a revelation. Some crucial examples are Joris-Karl Huysmans, La-Bas (1891), Talbot Mundy, Om: The Secret of Abhor Valley (1924) , and John Fowles, The Magus (1984)

Occult Detective: Occult detective stories are a sub-genre of the detective story. The main difference is that the occult detective is involved in cases involving ghosts, curses, and other supernatural elements. These days most occult detectives are really occult fantasies as their roots are more in the fantasy side then the detective side but there are exceptions. Theres also an interesting switch from occult detectives who abhorred the occult to ones who are active and enthusiastic practitioners (compare and contrast Dr Silence to Harry Dresden)

Occult Espionage: A rare breed, the espionage story turned to matters of the occult.

Crypto-thriller: Thrillers are characterized by fast pacing, frequent action, and resourceful heroes who must thwart the plans of more-powerful and better-equipped villains. Literary devices such as suspense, red herrings, and cliffhangers are used extensively. A crypto-thriller involves hidden history, secret societies, strange codes and puzzles. Think Da Vinci Code and all the rest.