They were fore-runners of graphic novels in which the reader chose the
path the hero would take on each page -- in the style of the Choose
Your Adventure series that came later -- then flip to the right page
for that option.

The books were illustrated with black and white line drawings. I
recall they were of a smaller format.

One of the books dealt with Chinese opium traffickers and might have
been set on a boat.

I recall the mood of these books was a bit darker -- there was murder
and mayhem.  I think they were British but I'm not certain.  I'm
guessing 1974-ish.

Anyone remember the names of these?

Thanks.