I think they're all worth a read. They do come in some variety of styles regarding how "serious" they are, but I found them all more or less entertaining.
I'd recommend at least Drew Wagar's Elite: Reclamation and Elite: Premonition.
Michael Brookes' Elite: Legacy too along with Lave Revolution (Allen Stroud), And Here the Wheel (John Harper) plus Out of the Darkness (T. James) are quite nice in deepening the setting of the game.
Others were more or less light-hearted scifi-romps in/around loosely E: D universe.
Some of the concepts & techs in these books are outdated / changed in the current game, cannot be helped as various writers had different backgrounds and expectations of what is to come, and FDEV hasn't stayed completely true to the writers' design document the authors had as their quideline back then.