I first read L. Sprague de Camp's Lest Darkness Fall nearly 15 years ago, and I've reread it dozens of times since then. It's one of my all-time favorite novels, and as I reread it again I thought about why I regard it so.
Probably the first reason is that it's a novel about time travel, one of my all time favorite genres. More significantly, though, is that it's a good time travel novel. One of the things I have learned from reading time travel novels over the years is that they're very difficult to get right, which is why there are many time travel novels and yet so few good ones. I think about something I read a while in a review of Stephen King's 11/22/63 (a book that, admittedly, I haven't read yet) that when writing time travel novels it's important to draft some rules and then staying true to them. I would add that it's important to keep those rules few and simple -- too many and the novel becomes less a story and more the author's convoluted explanation of how they think time travel would work.
De Camp avoids this trap by making just one rule and sticking to it. It comes at the very start of the novel (which is the weakest part of the entire book) when the main character, Martin Padway, has a far-too-convenient conversation in "modern day" (1939) Rome with a local who pontificates on his theory of time travel. It's clunky and dropped right in there, but it's effective in both outlining the rule and then moving on. The rule is straightforward -- that once a person travels back in time, the action of that travel splits time for them off into a new branch -- and it allows for the fun that follows.
And "fun" is the word for it. It is difficult to find a time travel novel that is written with as much brio as this one possesses. Once Padway is transported to 6th century Rome by a deus ex machina (subtlety doesn't rank among de Camp's gifts as a writer), he then proceeds to get to the heart of the book, which is to describe Padway's efforts to survive in the past. While Padway can communicate -- before his abrupt trip to the past he was a doctoral candidate in archaeology and thus knows both Latin and the events of the era -- he possesses nothing other than the clothes he is wearing and the items that he had with him at the moment of his unexpected temporal jaunt. His efforts to earn a living by building up a business is the main focus of his book, and while Padway may have a breadth of technical knowledge that would put the Connecticut Yankee to shame, de Camp entertains here by describing his protagonist's efforts to get the resources he needs from an indifferent populace.
Yet Padway soon realizes that it isn't enough simply to survive, and it is this realization that provides the transition to the second part of the book. For Padway is on the cusp of the "Dark Ages" (a concept that dates this novel more than anything else in it), and he decides that unless he wants to face the violence and misery soon to come he has to play a more active role in events. Here de Camp's prestated rule comes into play, allowing him to avoid the typically messy question of the impact of such changes and just focus on the narrative. And messy it is, as Padway proceeds with enormous panache, dramatically changing the history he knew in a variety of ways and facing considerable resistance from the locals in the process.
Were this book written by an author today it would probably get bogged down in the historical details or tangled in the webbing of relationships. That de Camp wrote at a time when genre novels were much shorter than now, though, is an invaluable asset to his pacing. The novel practically zips forward, with little space for contemplation, secondary characters, or subplots. It's this narrative drive that helps to zip readers through the short descriptions of Padway's devices or the politics of Gothic Italy. Combined with a sense of humor, it makes it an easy novel to read quickly and with enormous enjoyment. Lest Darkness Fall may not be ranked as one of the greatest science fiction novels ever written, but it is definitely one of the most enjoyable -- and one that I find that I can continue to enjoy no matter how many times I return to it.