: I swim about 1.5 km, six days a week and it usually takes about 33 - 38 min.
: Could someone suggest me a book or some kind of techniques (free style) to
: improve my speed.
You have to go backwards to go forwards.
Slow down your swimming a bit and concentrate on technique. It seems
strange, but you know when it's better. You should be able to feel your
hands actually doing work. Ascribe a keyhole pattern in the water.
Stretch out in front, and pull right through to the end of stroke.
Split your sessions up so that you get some rest along the way. To get
fitness benefit you don't have to go continuously. During the rest
remind yourself, "Stretch, keyhole, end of stroke".
A book. Read the worldwide bible of swimming, (but get it at the
library, it's pretty expensive):
Doc Counsilman's, "Competitive Swimming Manual for Coaches and
Swimmers", published by Pelham Books.
An absolute encyclopaedia of almost everything you need to know. The
freestyle section (complete with text, diagrams and underwater
photographs) starts on page 144 of the 1977 edition. A great read.