I read Letters From The Earth when I was about 11 or 12, which would make it
about 1964-5.  I think maybe it had just come out in paperback.  And it
might actually have been the very first Twain book I read, save Tom Sawyer.
I should re-read it, because I don't remember much about it now.  But I know
at the time it seemed remarkably frank (to me) about sex, and refreshingly
non-religious, or at least cheerfully blasphemous.  I suspect if I read it
now - given the intervening years culturally and personally, it would seem
pretty mild stuff.  But it was good stuff back then.

I think I even still have the paperback all these years later.  I envy you
reading it for the first time!

Carl