Lacuna, by David Adams
Lacuna, by David Adams, is a flat space opera.
Good features: Um ... the spelling and grammar are fine. It's original, not a rip-off of any particular other work I could identify. Some details are nice: see below.
The plot is pretty straightforward. Humans invent some useful space and space-weapon technology, in the lampshadey "Chekhov Arsenal". Unknown aliens show up and destroy three cities and say "Don't work on the jump drive on pain of death" (in bad Chinese), and vanish. So humans build three giant space-battleships to defend themselves, and tangle with the aliens, and there's fighting and some victory. (This is book 1 of a series.) This is not inherently boring, and a good writer could do a lot with it. But the quality of this space opera is in the details, and, though David Adams certainly tries, the details don't sing.
Case in point, with minor spoilers: the aliens don't want humans (or anyone but themselves) to use the jump drive. The reason, when they get to explain it surprizingly late in the story, is sensible enough. Once in a while, the jump drive accidentally creates a singularity that wrecks the star system it's in. It ate the aliens' home world, and so they don't like it at all. This is a good idea! But: the aliens use the jump drive constantly and freely. They won't let anyone else use it because of those singularities, but they use it despite that possibility. Also, humans insist on using it, without any qualms, despite knowing that it could destroy the Solar System.
Case in point: the jump drive cannot be used inside a gravity field. (Presumably, of a certain strength - everything is within a gravity field.) But Adams says that it can be used at Lagrange points. Now, there's a certain sense there - Lagrange points are where two gravity fields, say Earth and Moon, balance. But they aren't gravity-free. There are other gravity fields around, like the Sun's, and the galaxy's for that matter. (Also, wouldn't any point between starts work fine? Go a couple light-weeks from the Sun and the gravity is weaker than at any Lagrange point in the inner solar system. This would ruin an important plot point.)
But this could be forgiven if the characters were interesting. They're not. They're flat and flavorless, and when Adams tries to make them have a personality (as with the wisecracking science lead) it's even worse. They have lots of sex and a few conflicts, but there's no passion or interest there. The alien commander suddenly acquires a personal vendetta against the human captain, wanting to torture her by killing her entire species while she watches. OK, he has a good reason to be angry - she did just do a Pearl Harbor kind of thing on his military base - but his posturing and sneering is stereotypically and melodramatically evil, and it's unclear why he would care about her in particular. An honorable or cold-blooded enemy would make a lot more sense.
It's not the worst free e-book I've started by any means. But it's in the bottom 20% of e-books that I've finished, so it gets one Lagrange point out of five.