Learn your chords! The vast majority of music is tonal, and learning chords could simplify things from reading notes to reading chords, which is way faster.
I fully agree with pianoplunker that learning the chord system is a big step on the way to good sightreading and to the use of lead sheets. However, to take it a step further, how do we proceed from just playing block chords in the left hand which quickly becomes boring, to create a good accompaniment.
Find music you are able to sight read, this might be very very easy music compared to what you currently play. Start as easy as possible with your piece selection so you can read without problems. From here you can build, it takes a lot of humbling to lower your level but this is the best place to develop. There are methods to practice reading with more challenging pieces but in my professional opinion I would say studying hundreds of easier pieces, this will help you a great deal more to start out with.
What published selections would you recommend please?