Which biography of the Buddha is written in (or translated into) English, and is the most widely read (from a Theravada perspective)?
A book titled In the Buddha's words is "An Anthology of Discourses from the Pali Canon" by Bhikkhu Bodhi.
You can find it online as a PDF.
Its second chapter (only 25 pages) is titled "The Bringer of Light" and is a biography: it has section titles like, "The Buddha's Conception and Birth", "The Decision to Teach", and "The First Discourse".
Its advantage is that it's quoting suttas, so I guess that's "widely read" and reportedly autobiography.
There are many books on this topic, my favorite one is "Gotama Buddha: A Biography Based on the Most Reliable Texts" by Hajime Nakamura. It is based on a very thorough, I would even say fanatical, research into the primary sources (mostly Pali Canon) - and I found it by far the most detailed (two volumes!) and very well specified with references.
You can read AccessToInsight's "A Sketch of The Buddha's Life: Readings from the Pali Canon".
From a Theravada perspective, the life of the Buddha is given in the Vinaya and Sutta Pitaka. These texts are in an unknown language. The name that corresponds to 'the Buddha' in the canon is Sammasambuddho. A Bodhisatto becomes a Sammasambuddho as a consequence of achieving Sammasambodhi. The texts cover the life of Bodhisatto from birth to Sammasambodhi. Every single moment from Sammasambodhi to Parinibbana is accounted for in the texts.
The most comprehensive I have come across in English is:
- Maha Buddhavamsa The Great Chronicle of The Buddhas by Tipitakadhara Mingun Sayadaw
Also comprehensive but shorter books:
- Buddha and this Teaching by Venerable Nārada Mahāthera
- Thus We Heard:Recollections of the Life of the Buddha by Bhante Walpola Piyananda and Stephen Long