M.M. Lewis 
Infant Speech [PDF ebook] 
A Study of the Begiinings of Language

This is Volume XV of a series of thirty-two on Developmental Psychology. Originally published in 1936, this study looks at when speech begins in children. The sounds that a child makes during his first few months are so elusive and apparently so remote from anything that might be called language that any observer however interested in speech might well be pardoned for waiting until the noises become, at any rate, a little more obviously human. To persist in making observations one must be interested in the variety of human sounds merely as sounds, one must have faith in the continuity of growth, and in addition, perhaps, one must have something of that insensitiveness to ridicule which is found at its highest in the truly devoted parent.