It's the very best of Clarke's work: when he lets a good writer do the storytelling, and sticks himself to figuring out the logical conclusion of new technology

I don't know about other countries - in Switzerland there's something called 'tactile sign language'. It's a variation of sign language where the 'listener' is holding your hands to track your movements, and it's commonly used by/with people with impaired hearing and eyesight.

A more universal (and much easier to learn but much slower) variant is the Lorm alphabet where you use your right hand to draw letters on the 'listening' person's left hand. E.g. the word 'hello' would be: run finger down top half of pinkie, tap tip of index finger, run finger down middle finger to wrist (twice), tap tip of ring finger.

how people who are both blind and deaf communicate

You can spell, but that would slow down conversation significantly. I get frustrated sometimes when people talk too slow, so I think I would just end up blurting it out.