Sound behaves in two ways in your room. The bass frequencies, roughly up to 300Hz, reverberates around the room; and the higher frequencies, which reflect like light in a mirror. They need separate consideration.
Bass issues are the most important to address. Once done it will also improve the higher frequencies.
This site will give you some idea of the bass issues in your room:
https://amcoustics.com/tools/amroc
Put in your room measurements and it tells you the possible problems. If you click on the link, top right, 'More about room modes...' there some information about this.
As pointed out above, speaker and listening chair placement is important. This is what I use:
http://www.barrydiamentaudio.com/monitoring.htm
And measuring is essential to do the job properly. REW again has already mentioned. I've used this for many years and still don't understand most of it but it works well.
So the principles are:
1. Position speakers and chair as best as possible.
2. Use room treatment to address problems, particularly in the bass region, but perhaps also reflections from the higher frequencies (there are a range of views on this which are worth exploring).
3. Careful use of DSP/EQ to complete the bass issues, and perhaps, very careful use to adjust the higher ones if you have problems there.
Use copious measurements as you move along this route, using frequency responses and decay time measurements - waterfall plots, spectrograms and others perhaps.
But, ultimately, don't get obsessed with measurements. They are just a guide. The real test is 'does it sound good to you'.
Books? Floyd Toole's 'Sound Reproduction, Third Edition'.
And perhaps this:
http://getbettersound.com/index.php
There are other solutions that you may want to consider and which I know little about. Using a number subwoofers to counteract the bass problems; and using only DSP. There are discussions around on these subjects.