Reddit reviews Sayings of the Buddha: New Translations from the Pali Nikayas (Oxford World's Classics)

I'm afraid I can't answer your question directly, but I can help contextualize it historically and hopefully point you in the right direction.

Like u/ikid_ikid noted, the 6th century BCE didn't have strict national borders in the sense that we do nowadays. That part of India/Nepal was comprised of several smaller kingdoms, most notably for our purposes the Magadh. As you may know, the Buddha was born as a prince named Siddhartha in the Sakya clan (Sakyamuni translates roughly to 'sage of the Sakyas'). His hagiography depicts him as unusually skilled in many areas, including language, and this certainly would have helped him reach a larger audience in his teachings. It's also worth pointing out that, after his enlightenment at age 35, he taught for 45 years before his death. That's a long time, and despite the fact that he travelled by foot he was pretty mobile. If you look at this slightly cluttered map of the important sites in his life, you see how active he was.

However, Sakyamuni Buddha was not the only person who spread Buddhism. Far from it. To understand how Buddhism spread so quickly throughout northern India (and eventually much of South and SE Asia) let's look at two important factors. The first is the First Buddhist Council, which convened the year after Sakyamuni died. The council, comprised of 500 arhats (englightened ones, who had been taught by the Buddha or his close disciples) convened and standardized many of the monastic rules and teachings of the Buddha (Pali: dhamma, Sanskrit: dharma). With everyone on (theoretically) the same page, the arhats could spread the Buddhadhamma wherever they went.

The second important factor is the reign of King Asoka pdf, in the 3rd century BCE. Asoka was arguably India's greatest king ever, and he was the first ruler to unify most of the Indian Subcontinent. And he was a Buddhist. (Actually there's some speculation about his personal beliefs, but certainly promoted Buddhism as a matter of state). This was great news for Buddhism, which was popular but still relatively local and competing against plenty of other religions (Jainism, Ajivikia, the Brahmanism that would eventually become hinduism, to name a few). Asoka event sent his daughter to Sri Lanka, and established Buddhism there.

In my opinion, the First Buddhist Council and the reign of Asoka are the most important factors in understanding how Buddhism spread the way it did. However, there are plenty of other factors that I won't go into here. For example, if we look into the suttas in the Pali Canon, we see thatthe Buddha was extremely well-connected in his lifetime. This is not only due to his royal lineage but also to his wealthy and powerful lay disciples such as King Bimbisara. On a more societal level, the Buddha lived in a time of great social upheaval and urbanization. This may have lead to an increase in mobility and openmindedness that he might have made use of. Finally, if you want to get supernatural, the suttas often mention that the Buddha had 84,000 teachings -- which we might read as an arbitrarily high number -- for any individual's abilities and disposition. He also had plenty of "magical" powers. Though I don't specifically know if these allegedly allowed him to communicate using other languages, I would not be surprised to learn that they do.

As for sources, I apologize because I do not have my books with me at the moment. I've also taken some of this information from undergrad lectures, and therefore don't have specific references for them. As to general readings, I highly suggest reading suttas from the Pali Canon. The collection edited and translated by Rupert Gethin is the best I've read, and the preface in particular addresses some of what I've discussed. Gombrich's book about Asoka is very well-respected, as is Conze's introction to early Buddhism.

Highlights from my library...

At introductory level, Skilton A Concise History of Buddhism is a good overview of Buddhism from an historical perspective.

Foundations of Buddhism by Rupert Gethin is a fairly good modern overview of Buddhist ideas.

Kalupahana A history of Buddhist philosophy - slightly eccentric, but some excellent coverage of Buddhist thought. Stops short of Tantra. However, supplement with Samuel The Origins of Yoga and Tantra.

Snellgrove Indo-Tibetan Buddhism covers late Mahāyāna and Tantra; theory oriented. Perhaps a but dated now, but non-sectarian.

For early Buddhist texts, Gethin's selections from the Pāḷi Canon is probably a better bet than Bodhi's massive tome. It is more focussed. If you get into Pāli Suttas then get the Nikāya translations, starting with Majjhima Nikāya (the most readable). Another lesser know anthology is by Glenn Wallis Basic Teachings of the Buddha. Slightly idiosyncratic, but a very interesting perspective.

For a traditional anthology of Mahāyāna texts try the much neglected Śikṣamuccaya by Śāntideva (8th Century). I have a different edition to this link, so maybe look around.

Tantra doesn't lend itself to anthologies. I think probably the best introduction to Tantra is Hakeda Kūkai: Major Works, though it may not entirely stand alone, you'll at least know what the right questions are. I've never found any book by a Tibetan Buddhist to rival Kūkai for clarity of exposition about what Tantra Buddhism is.

In terms of practice, Buddhism is often sharply divided by sectarianism and so books on practice tend to be narrowly focussed along sectarian lines. And even on single practices within sects (such as books on Theravāda approaches to breath meditation only; Zen approaches to koan practice and so on.). Note that having asked for a comprehensive book, most of the suggestions are sectarian and ignore centuries of development of Buddhism in other directions.

Buddhism also often suffers from a theory/practice disconnect. A lot of our theory/doctrine has no practical application; while some of our practices are poorly understood and communicated.

The fact is that if you take one sect at a starting point, you will find much of what you learn contradicted when you shift to a different sectarian point of view. Sects tend to present their sectarian point of view as Buddhism without ever letting on that other brands are available. So a comprehensive outline of Buddhism is almost impossible without fudging a lot of conflict and contradiction.

Books of anybody named Gethin are not the primary religious texts in Buddhism. There is a Rupert Gethin thats written books about Buddhism though, according to Google.

I think this must be it
https://www.amazon.com/Sayings-Buddha-Translations-Nikayas-Classics/dp/019283925X

You might want to read just one of the four Nikayas, or an anthology like Sayings of the Buddha or In the Buddha's words.

On the Mahayana side, it is better to start with the books of modern teachers like others have recommended. My personal recommendations are any book by Khenchen Thrangu Rinpoche and "The method of no method" by Chan master Sheng Yen. Mahayana sutras can be underwhelming or confusing to the beginner - I can recommend the Prajnaparamita sutra (i.e. the Diamond sutra or a longer version.)