From the study you posted it seems to me that you are hand-tied to reference. However, you do have solid execution of values and I think you could do great stuff if you can break away from ref.
I have found that to successfully draw from imagination, one must have a solid foundation of perspective (specifically proper ellipses in space- here's a tut on eye level that mentions this principle-
eye level tut) and a solid knowledge of lighting. Otherwise, it's rather hard to imagine how everything will look. Construction drawing is a great place to start. I recommend drawing shapes as if you could see through them, using lines to show how the form fits in a spacial plane relative to the horizon.
I wish I knew a great book to recommend when it comes to lighting. I gleaned what I know from several college art courses and experience over time. I am just barely getting to where I can make up scenes with decent believability. The main advice I can give is treat everything as if it is a plane in space and think of how the light would hit that plane. Also, know the difference between cast shadow and form shadow and don't be afraid to render hard cast shadows. This has helped my work immensely!
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