To begin with, I have translated a nice essay on the Tibetan script written by the Tibetan polymath Dungkar Losang Khrinley. It appears in his encyclopaedic Great Tibetological Dictionary, which was published in 2002. Though the opinions expressed here do not necessarily reflect my own, Dungkar has written an excellent introduction to the features of the Tibetan script, and also made a strong case for its ongoing relevance.
My translation of a traditional account of the invention of the Tibetan alphabet from the earliest extant source of the story, the 11th–12th century Pillar Testamant (Bka’ chems ka khol ma). For more information on this text see the THDL site, which has a contents summary here, and Leonard van der Kuijp’s discussion of the text here. Coming as it does some 400 years after the invention of the Tibetan script, this shouldn’t be read as an accurate historical account. Its value is in being the first version of popular story of Tönmi Sambhoṭa, and in its detailed (if not necessarily accurate) account of how the Brahmi alphabet of India was transformed into a Tibetan alphabet. Note that we see here a misunderstanding that became ubiquitous in Tibetan writings on the alphabet: that ca, cha and ja were not found in the Indian alphabet and had to be specially created, when in fact it was tsa, tsha and dza.