Insert a string into a string

Is there a special function to insert a string into another string ?
For example I want to exchange the german "Umlauts" =E4,=F6,=FC with ae,oe
and ue in a string.
I also wonder if there is a function to get the alphabet of a string ?
Do two strings share the same alphabet ?

P.S.: What book can you recommand your C and/or C++ ? Don=B4t say
Kernighan & Ritchie. Should be useful not only to learn but also to
look up things.

Thanks a lot