I recommend you try to return that book "Let us C" it is so outdated that it shouldn't be used. Here is a quote from one of the reviews on Amazon.

it's still about 16 bit integers and turbo c/c++
Turbo C, enough said?

The second book is possibly a good reference book for the language, although it is also getting a little old, but I don't recommend trying to use it to learn the language.

From this sites C book recommendation page you may want to look into C Programming: A Modern Approach, 2nd Edition

Jim