The full huge range of sounds available from synthesisers means that the subject is enormously broad. Typical keyboards behave differently depending on the virtual instrument you are playing.
For example, a piano patch isn't likely to respond to aftertouch, and might not respond to pitch bend; a strings patch will respond to pitch bend, and might swell if you press harder while sustaining the note.
Electronic keyboard use falls into three main camps:
- standing in for a traditional keyboard instrument: piano, harpsichord, pipe organ, Hammond organ, electric piano, mellotron
- standing in for other traditional instruments: flute, trumpet, string section
- sounds with no traditional equivalent
For playing in the style of piano/organ, learning materials for those instruments will be perfectly transferable to the electronic keyboard. Practising piano pieces using the piano patch on your electronic keyboard will always be beneficial. Learning organ pieces with an organ patch will also be useful, and you'll be able to compare the different playing styles.
Many sounds naturally fit "piano style" playing, or "organ style" playing. For example, a glockenspiel sound is percussive and decays; so a piano style is more applicable. A flute sound is sustained, so an organ style fits better.
Note that some pianos have very light keys too.
I'm not aware of well-known teaching material for imitating other instruments. The skills learned playing piano/organ parts will be very transferable to this. But you'll also want to bring in aftertouch, pitch bend and modulation, for volume, vibrato, and bend effects. I suppose one way to practice is to find a score for (say) trumpet, along with a recording of a real trumpet playing it, and try to imitate that.
Music technology magazines like Sound on Sound sometimes have articles on imitating other instruments. There's an awful lot to know, if you want get it right -- for example, playing a guitar sound such that you never play two notes together which would be on the same string.
For purely electronic sounds, the sky's the limit. The skill of creating synthesised sounds is separate from the skill of playing them. A lot of the time, the parts you hear in records are not difficult to play -- or when they are, they're sequenced.
For the piano and organ pieces, at least, you should be able to follow structured courses intended for those instruments; so working through a series of piano exams (whether or not you actually get yourself graded) will work. For imitating other instruments, you may find that a piece that's easy on one instrument, is harder on a keyboard because of the key or the hand stretches. Wait until you can recognise whether a score is within your difficulty level by looking at it.