Giant from the Unknown

The citizens of Pine Ridge, a small California town set in the mountains, are concerned about a series of livestock mutilations that have taken place. A local man named Harold Banks is found dead and had been killed in a manner similar to the livestock. These incidents had been happening around an area known as Devil's Crag and the local sheriff orders everyone to stay away from there. Some citizens speculate that the deaths have a supernatural cause and a local Native American man known as Indian Joe confirms their fears of a curse before being driven away from town by the sheriff. A local geologist named Wayne Brooks enters town and is told of the death by his friends Anne and Charlie Brown, two young siblings living in the area. The sheriff has heard about a confrontation that took place between Brooks and Banks several days before and is suspicious of Brooks. While the sheriff is informally questioning Brooks about the death, Dr. Cleveland and his daughter Janet enters town. Cleveland is planning on conducting archaeological research in the area. While Brooks helps Janet get supplies at the store, the sheriff warns Cleveland about the recent murder. Brooks informs Cleveland that he was formerly his student at college and offers to be his guide in the area. Brooks, Cleveland and Janet agree to have dinner at the lodge later that day.
While waiting for their dinner at the lodge, Brooks and Cleveland discuss the recent killings. Brooks tells Cleveland that the killings have taken place near Devil's Crag and that the local Native Americans have a legend that the area is cursed and one day evil spirits will rise in that area. Brooks also informs Cleveland that Native American artifacts have been found in the area and that he has a collection of them. Cleveland asks to see the artifacts and Brooks agrees to show them to him. Cleveland then tells Brooks about his reason for coming to the area. He is searching for the remains of a Spanish Conquistador expedition he believes reached that area 500 years ago. The expedition was lead by a man named Ptolemy Firello but the specific group Cleveland is looking for is a small group known as the "Diablo Brigade" that split off the main group. This group was led by a giant named Vargas, also known as the "Diablo Giant." Janet then joins the two men and agrees to accompany Brooks to the local cinema after he takes Cleveland to see his laboratory.
Brooks takes Cleveland and Janet to his field laboratory to show them his artifacts. While Cleveland is examining artifacts, Janet cries out is fear. She had opened a box and found a small lizard inside. Brooks tells them that he found the lizard inside a rock and that the lizard had been in a state of suspended animation for a very long time. Brooks and Janet then leave to go to the cinema while Cleveland continues examining the artifacts. Indian Joe can be seen peering through the window.
As soon as Brooks and Janet arrive back at the laboratory from the cinema, Cleveland excitedly calls them inside. He had pieced together broken fragments into a cross. He theorizes that the Conquistador group he is after had influenced the natives of the area hundreds of years before and that is why they had made a cross. Brooks offers to take them to the spot where he had found the cross at in Devil's Crag and Cleveland agrees to go there the next morning.
The next morning, Brooks, Cleveland, and Janet leave town to go to where the broken cross was found. The sheriff sees them and follows them in his police car. When the group reaches Devil's Crag, Janet sees someone watching them in the undergrowth. The two men dismiss it as just a deer. The sheriff pulls up just then and chastises Brooks for leaving town and tells them Devil's Crag is off limits due to the danger. Cleveland produces a permit from the Commissioner of Public Lands that allows him to do his research and assumes full responsibility for Brooks. Cleveland also assures the sheriff that they are armed and can defend themselves from any danger. The sheriff then leaves and the group set up their camp.
The next day, while Brooks is examining the area, Indian Joe fires his rifle in Brooks' direction. Indian Joe then approaches Brooks and tells him that he is hunting rabbits. Indian Joe asks if they are there to rob Native American graves and Brooks assures him that they are only after Spanish artifacts. Indian Joe agrees to hunt elsewhere and warns Brooks that the place is evil.
Brooks returns to camp and tells Cleveland that the area is changed from how he remembers it. He theorizes that a recent electrical storm disturbed the area. The men discuss their plan to use metal detectors to search for Spanish artifacts and avoid Native American graves. They leave Janet to take care of the camp.
Brooks and Cleveland use their metal detectors to search the area all day without success. Janet encourages Cleveland to give up the search and he sadly agrees to do so. She wants to try to use a metal detector before they pack up camp and does so until accidentally detecting something. The group excavates the spot and find Spanish artifacts such as armor, weapons and bones. Cleveland plans to bring the artifacts to the museum and publish his findings. Brooks finds rock similar to the rock the lizard was entombed in and the handle of a massive axe he believes belonged to the "Diablo Giant." Brooks is forced to go back to camp because an electrical storm is beginning. As Brooks leaves, the "Diablo Giant" rises from the forest detritus.
The next day, the group examines the spot where Brooks found the axe. The axe is gone but they find giant-sized armor and other artifacts as well as a large indentation in the earth. They discuss the possibility that Vargas was in a state of suspended animation like the lizard and is now alive. At camp, the group arranges the artifacts on tree branches. Charlie Brown approaches the group and sees the artifacts. Brooks asks him not to tell anyone about what he has seen there because lots of people coming to Devil's Crag to see the artifacts would make their work impossible. Charlie agrees and leaves.
Wayne Brooks accidentally reveals and proves the theory of suspended animation when he finds a frozen lizard in a mountain rock which comes back to life. Dr. Cleveland has a passion of writing a book about Spanish Conquistadors who came to the region. The book targets the history of a legendary giant Conquistador named Vargas whose remains Dr. Cleveland, his daughter, Janet, and Brooks search for in an archaeological dig containing ancient armour and weapons. Later that night, a lightning strike revives the body of the 500-year-old Vargas, who stalks the three searchers and eventually kills a young woman. Sheriff Parker, the town Sheriff accuses and arrests Wayne for the murder of the young woman because a medallion from the search was found in the girl's hand. But later, it is revealed that Vargas is roaming around the forest causing another brutal death. The local town men, with the help of the three searchers and the sheriff go out to find and kill the giant, which causes more damage and deaths. Wayne finally kills the giant by causing him to fall off a bridge and into a waterfall.

Question: What is the subject of Dr. Cleveland's book?
Answer: Spanish Conquistadors