Brent de Chene 
The Historical Phonology of Vowel Length [PDF ebook] 

Data from a variety of languages are offered in support of the claim that although there are several processes by which languages commonly add to an already existing stock of long vowels, there is only one mechanism by which a language without a distinction of vocalic length commonly introduces such a distinction. This mechanism is the coalescence of vowel sequences, typically after loss of intervocalic consonants. This book examines vowels lengths, their differences and their effects on language.