| | why membrane is asymmetrical ?
If "why" referes to the mechanism by which such asymetry is created, any
book on molecular cell biology (e.g. Albers et al. or Lodish et al.)
will answer the question. Look for phospholipid biosynthesis (and also
membrane protein biosynthesis).
If "why" referes to purpose, then because the environments on both sides
of the membrane are different (cytosol vs interstitial fluid) and the
two leaflets of the membrane have different tasks. Therefore they have
different composition. The cited books will also enlighten you on the
details here (check the chapters on transmembrane transport, receptors
and immune system).