"Masters of Time" is a 1950 novel by famous science-fiction writer A.E. van Vogt. Although, (somewhat ironically, given the title), the date of the book is not clear. My copy was published in 1967, but the copyright for the author was in 1950, while it had a copyright for 1942 and 1944 for Astounding Science-Fiction. This story might have been a fix-up, although it could have used more fixing up.
Norma is a middle-aged woman (I think she is like 35 in the story), who wants to kill herself because she is old. But at the last moment, a man rescues her. It turns out, though, that that man is a member of a time travelling race of super-beings called the Glorious, and we know this Dr. Lell is up to no good, because he has dark skin and "slant eyes, like a Chinese." He mind controls Norma, who writes a letter to her ex-boyfriend, a scientist named Jack Garson, who goes to investigate and also gets kidnapped, and there is a depersonalization machine, and it turns out that time creates a new reality millions of times a second, and that a group called "The Planetarians" who are using a time-energy barrier to destroy the Glorious ship. There is an Observer with "tentacles", and Norma realizes that the aliens are controling her with a small pen that changes her age from young to old, but she unscrews it, and then there are also a third force, called "The Wizards of Bor" who have their own agenda, and also Greek hoplites and Roman legions that are time displaced soliders and...
Okay, so the thing is, I read a lot of science-fiction, a lot of it pulp. And sometimes I find myself losing the threads of the story when I am reading an explanation of the internal politics of a galactic empire or the mechanics of a hyperspace drive. But this story just didn't make sense. The storytelling, world building and characterization just don't make sense, and so many concepts were thrown out with no connection, that I got lost. The strange thing about this is that A. E. van Vogt was not an anonymous pulp author spinning a fast book, but one of the pillars of "Golden Age" science-fiction. (Although apparently, he was not universally lauded). I am also reading this book almost 80 years after it was written. Some of the ideas in the book (time displaced armies, alternate timelines, brainwashed armies, etc) were probably startling new when it was written, but I have had a good fortune to see them executed in much better ways since. But despite giving it points for the early introduction of concepts, I couldn't follow this book enough to enjoy it.