Speaking of no-charge books, I found The Solomon Kane Mythos by Robert E. Howard at no cost on iBooks, published by Ozymandias Press. It's 322 pages on my iPhone (which probably means it's about 100 real pages in a book).
I'm about 60 pages in and Kane has pursued Le Loup, the bandit leader from the first story, from France (which I will admit confused me, because I associate Puritans with New England, not France) to Italy to somewhere in Africa. Once we're in Africa, Howard gets kind of racist in his descriptions (unfortunate, but entirely expected) but that's also where the real weirdness starts, with corpses rising and an angry gorilla killing the person who killed its mate.
More from same book, but different story: The Moon Of Skulls!
“The Englishman gazed, fascinated though repelled. Not even in the courts of Europe had he seen such grandeur. The chamber and all its accoutrements, from the carven serpents twined about the bases of the pillars to the dimly seen dragons on the shadowy ceiling, were fashioned on a gigantic scale. The splendour was awesome—elephantine—inhumanly oversized, and almost numbing to the mind which sought to measure and conceive the magnitude thereof. To Kane it seemed that these things must have been the work of gods rather than men, for this chamber alone would dwarf most of the castles be had known in Europe...”
Classic scene of a lost ancient civilization, in this case in a cave that can only be reached through the jungle.