I found the theory from Aletha Solter by reading an article by a spanish therapist (Angel Alvarez) about the crying needs of a baby.
Being recently the mother of a "high need crying" baby that had a very diffcult delivery and being surrounded of baby counselours that advocated that a baby should never cry, I was trying together with my partner to mitigate all the crying sources for my baby. But I always felt inside that there was something wrong that a baby can't cry, and after a while I had the impression that was something wrong with my baby as well. Finding this theory and the reading this book is helping me to create a new relationship with my son, he is much more alert and happy, although the intense crying sometimes. I feel much more confident as a mother and I have earned a better perception of a baby state of mind. The book is very well written and it is very comprehensible.