I have very vague impressions of a book I must have read as a child/teenager in the 1980s (so probably published in the UK in the 1970s or early 1980s).
It featured a man who had been technologically enhanced with special powers that he could activate only for short periods (probably because they took a lot of energy), possibly by tonguing one of his teeth. They gave him:
- Much faster reactions; everything around him slowed down.
- Greater strength/speed.
- Metal fingernails/claws.
- Possibly some sort of enhanced vision, too.
I think it described him engaging these powers as:
He activated. or similar. He may have been a mercenary type, who had paid a lot for them; they were rare or unique.
I don't recall any plot or other characters; it was probably a techno-action story. (No magic, armies, warrior races, telepathy, vampires, or mutants, and I don't recall aliens or space travel.) It may have been part of a short series of 2 or 3 books.
I'm afraid this isn't much to go on! I'd completely forgotten about this book until recently, and my impressions are very hazy, so any of the above could be wrong. (I remember enjoying it, though.) I've not found anything I recognise on this site, or others; nor in lists of adult and young-person's SF for that period (so it probably wasn't a big-name author). Does it ring any bells for anyone?