Triangular buildings aren’t so very unusual, triangular books, on the other hand, are less common. This example is from the Manly Palmer Hall collection of alchemical manuscripts at the Internet Archive, not only a triangular book but one where most of the pages are written in a symbolic alphabet. A reviewer supplies the following details (which may not be accurate so the usual caveats apply):
“No. Soixante & Seize” de la collection maconnique du F… Ex Dono Sapientissimi Comitis St. Germain Qui Orben Terrarum Per Cucurrit ca. 1775. Hogart MS 209.
This manuscript bought from Frank Hollings, a London antiquary, after 1933 (he apparently was unaware of the Hauser St. Germain manuscript) came from the occult library of Mme. Barbe, who had it from the bibliographer Stanislaus de Guaita, who in turn bought it at the auction of the library of Jules Favre. It is a copy made from one of the magical texts in the possession of St. Germain by the owner’s permission. A number of such copies were executed for the members of his Masonic lodge in Paris, and the following manuscript, as different in style as it is, may be one of the copies too. It is unclear in both cases whether the Comte St. Germain wrote the magical formulae or owned a copy of an ancient text. This manuscript was made for Antoine Louis Moret, a French emigre to America active in Masonry and in politics.
Does the 76 on the cover refer to the year the book was made, or does it have some other significance? One of the meanings assigned to the number 76 in the Sepher Sephiroth is “Secret, put away; a hiding place”, so the latter is a possibility. See the entire volume here.