[AdiDex]

I've never seen any example of Resonance in conjugated Alkynes. Does Resonance exist in C≡C system?
For example
  .

Or
 
Diacetylene

It is certain that -CN group is a -M group but what about -C≡CH? Is the Bond length between C₂ and C₃ shorter as compared to typical C-C bond?

[Corribus]

Resonance exists, but it tends to be more complicated due to more dramatic bond alternation in conjugated oligo-acetylenes. You get cumulenic (charge transfer) forms as well.

[AdiDex]

Can you please tell me any book or any Link ,where I can read more about it ? What do you mean by "dramatic bond alternation" ?

[Corribus]

Basically, triple bonds are shorter than single bonds. For a true resonance structure, all the bonds should be equal in length. For long (like, infinite) chains of alternating triple and single bonds, the alternated form is lower energy. This symmetry breaking is called a Peirels distortion (https://en.wikipedia.org/wiki/Peierls_transition) and is common in conjugated polymers. It is a reason there is an optical bandgap in polyacetylene (conjugated polymers are semiconductors rather than conductors, basically). Anyway, the energy difference between the bond alternated form and the unalternated resonance form limits the degree of conjugation/resonance (whatever you want to call it) as the number of sequential ethynes increases.