+1 on Harry's comments above. I haven't invested the time to get to where I want to be with fretless bass guitar yet, but I did study double bass and can share some insights from that. As Harry said, you want to be playing without looking at the neck. On double bass, I worked through the standard material.The Simandl books and 30 Etudes, the latter of which can be had with a CD for piano accompaniment, which is very helpful for improving intonation because you can hear your pitch relative to the piano. Sight reading in general is good, because you will look at the music instead of the neck. Beyond the 30 etudes, there are piano accompaniments (CD and midi) to the Vivaldi Cello Sonatas and other classical repertoire available. It might seem strange to practice this music when you want to play other genres. But I found that sight reading and playing with accompaniment is very effective at developing technique in general and especially improving intonation. You wouldn't necessarily need to play the double bass material, as long as you were sight reading and had a piano accompaniment for pitch reference.
Also, when you play a fretless instrument you train your brain to constantly listen to intonation and your left hand to constantly adjust pitch in real time. This focus on pitch will develop your brain and I think you will hear music differently as a result of this. I did.