Using multiple permutations to strengthen the security of a cipher In one book it says


  a set of permutations with the composition operation is a group. This
  implies that using two permutations one after another cannot
  strengthen the security of a cipher, because we can always find a
  permutation that can do the same job because of the closure property.


If I try to understand the bold part, is below explanation correct?

Assume you have a set of all permutations of "abc", that is:
Your set is: "abc" ,"acb", "bca", "bac", "cab, "cba".
Let's take "abc".

Permute it once and get say: "cba" (Let's say permuting once means you encrypt it).

Now let's assume you want to strengthen above permutation by permuting it once again (encrypt it once more), e.g. now you permute "cba" and arrive at "bac".
In theory one could have arrived at "bac" from "abc" in a single permutation too (in a single encryption), thus additional permutation didn't really make much sense from this point of view. Because it says basically what you can do in two permutations you can effectively also do in a single permutation.
At component level, such as Sboxes, you are essentially correct. This is why an (unkeyed, fixed) SBox is never composed with another SBox, without some kind of other mechanism, such as key bit addition, or a permutation of bits between adjacent SBoxes [as suggested by @Paul Uszak]. 

Note that above, I interpret the word "permutation" as a fixed, unkeyed map. When keys are mixed in, and we view the full cipher globally things change drastically. In this case, the permutation which is the composition of two (or three for triple DES) permutations is extremely difficult to determine since


the search space is so large
there are no shortcuts, the permutations are not presented in lookup table form, plus random keys are involved so we now have essentially random permutations.