Empire From the Ashes (find it for free here) is a trilogy-turned-collected-volume that is one of the earliest works by David Weber, who is probably best-known for his Honor Harrington series. (His earliest credit on Wikipedia's list of his published works is for some book in 1990; his next is Mutineer's Moon in 1991, which was the first part of this story.)
It's pretty good, especially since you can read it for free, but its quality in and of itself is not specifically the reason why I felt like reviewing it.
No, that would be because it reads like a checklist of his favored plot points and story features.
Here's just a few of the things I've noticed, based on things entirely off the top of my head and only from books of his that I've randomly read:
Numerous and extremely powerful ancient aliens who nonetheless never advance technologically in time periods spanning millions of years. (This book has the Achuultani; the later Safehold series, of which I have read one book, has the Gbaba, which may be an even more extreme example of this. It should be noted that Weber does not use many advanced aliens in his works.)
Humanity has fallen back from being extremely advanced spacefarers to stuck on a single primitive colony world. (Notably, Empire From the Ashes has two worlds shown which this applies to, and it actually happened at least two more times; this is the central precept of the Safehold series.)
A contrast between huge-scale combat between spacefaring civilizations and smaller-scale warfare in medieval/Renaissance-class cultures. (Earth's moon turns out to have been destroyed by Dahak, a massive warship from an ancient human civilization, in order to camouflage itself as the moon, and it was a ship tasked with picket duty [it was also stated that it could have vaporized Earth itself]. On the other hand, the planet Pardal was stuck in a sort of medieval stasis for about 45,000 years. Safehold, again, has a lot of this; a certain amount of it can also be found in the Empire of Man/Prince Roger series.)
Characters are apparently killed and believed missing for a period of months or years, but they're protagonists and still alive, struggling for the entire time to make their way back with limited resources. (Five of the characters were nearly assassinated by the self-destruction of a ship similar to Dahak, mentioned above, but the ship jettisoned them beforehand because it had special programming not to harm them [it's too much of a story to explain], and they spent about two years traveling to the nearest solar system, and found that it was inhabited by primitive humans who were controlling a quarantine system [ditto], but which had what they needed to get home if they could just get a hold of it. In Empire of Man/Prince Roger, a trilogy's worth is about the journey across a planet with varying levels of cultural sophistication to get a spaceship, only to discover that they're now wanted outlaws. And in Honor Harrington, the titular character was once thought dead but managed to escape to a prison planet, where she was able to stage a prison break.)
Religion is used as a tool to hold back technological development. (I'm not going to go too deeply into this, but it's a feature shared, again, with Safehold. It's almost as if the Pardalian sequences intrigued Weber enough that he wanted to do more with them than he had space for, and so he decided to write an entire series about a variation of them. It should be noted that these religions were generally ironically paired with technology to help maintain them.)
Extensive explorations of the implications of differences between the less advanced cultures and both our own counterparts to them and the advanced cultures encountering them are a must. (The planet Pardal, stuck in medieval stasis, had a number of native animals that made for differences in military development between their cultures and ours. Safehold, surprisingly, doesn't explore this aspect much [as far as I recall], but it's a pretty big deal in Empire of Man/Prince Roger. As for advanced versus less advanced, there's usually a lot of adding to the primitive side's tech base in sustainable ways and limited, careful use of the advanced side's limited technological resources and transhuman abilities. It should be noted that this was usually hidden in Empire From the Ashes and the Safehold series and given a mystical explanation when revealed to the "primitives," but in Empire of Man/Prince Roger, they don't bother doing anything but using it. Of course, the people of Marduk, who happen to be the only nonhuman example of "primitives," know a bit better than those others, but still...)
There are probably (well, definitely, actually) plenty more comparisons that could be made, but I don't want to list everything because 1) you could read it for yourself quite easily, and 2) I'm tired right now and I think I'm pretty close to done.
Empire From the Ashes is definitely a good read; if you don't mind reading a long text on the computer and like military science fiction, it's a darned good deal.