C.J. Cherryh’s ‘Company Wars’ novels.
They are solid, hard SF. The characters are very real and human, and the endings are upbeat.
There is always a sense that basic physics matters. The only really new technology is the FTL drive, and even that is a complex technology with its own issues, hard on the physiology, and travel takes weeks of real time.
Even though there is the FTL drive, there is no artificial gravity, and ships and stations have to spin or they are in null-G. Life-support systems are always a background concern. They cannot be taken for granted. Maintenance of complex systems is ongoing. You have to buy flour and bake bread in ovens when the ship is not going to be in null-G. If you are 3 light minutes away, then you have to wait 6 minutes for a response to your message. Relativistic time-dilation due to travel can affect relationships. Etc.
Even villains are real people with limited vision just trying to get by, or trying make a quick buck, or power-hungry. There are no superheros. In the war, both sides have genuine, but incompatible, points of view. Both sides have political infighting, factions and parties.
The books all have characters you actually care about, and (mostly) they survive and do well by the end. The books can be gritty and harsh in places, but they are certainly not depressing.
Downbelow Station (1981) – Hugo winner, Locus SF Award nominee
Merchanter’s Luck (1982)
Rimrunners (1989) – Locus SF Award nominee
Heavy Time (1991)
Hellburner (1992) - sequel to Heavy Time
Finity’s End (1997) – Locus SF Award nominee
Alliance Rising - due to be published January 2019
To which I’d add
Cyteen (1988) – Hugo and Locus SF Award winner
(Cyteen is in the same universe, just not during the war. It covers the darker side of cloning, brainwashing, and an underclass of workers and soldiers, but looked at in a highly unconventional way. It is a bit claustrophobic and depressing, but still has a fairly positive ending.)
Downbelow Station sets up the universe, but you can read them in any order (except that Hellburner is the sequel to Heavy Time).
Hellburner is one of the best depictions of inefficient military bureaucracy I’ve ever come across - it’s also pretty funny.
Finity’s End focuses more on kids and young adults in a ‘merchanter’ family ship.
Tripoint is about bad parenting and rape, and - strangely enough for a book by a female author - though the book revolves around a disputed rape which happened 20 years earlier, by the end you feel that the guy actually has a valid point of view. The main character, who was born as a result of the ‘rape’, eventually has a choice of whether or not to stay on the borderline-illegal ship run by his somewhat dodgy father (no spoilers, but it’s remarkably interesting, with a very upbeat ending).