There are so many good ones, it's impossible to cover them all. However, I'll post some of the ones I like.

One of the earliest accounts is the "Periplus of the Erythraean Sea," which was written by an anonymous author back in the 1st century C.E. This document is an intriguing guide to the Indian Ocean region (we're talking India, Persia, Arabia, Egypt, and Horn of Africa) and provides insights into the peoples, cultures, and products found in the area. It was written from a Greek perspective and serves as a valuable source for understanding the cultures of the Indian Ocean basin.

Other ancient accounts include the writings of the Greek historian Herodotus, who in the fifth century B.C.E. discussed Egypt and Ethiopia in his book, The Histories. He provided descriptions of the people, customs, and religion of the two countries, as well as the trade between them. Similarly, the Greek geographer Strabo wrote about Ethiopia in the first century C.E. Strabo's account of Ethiopia includes details of the geography, people, and customs of the country.

Right next door, the Roman historian Tacitus wrote about Britain in the first century C.E. in his book, Agricola. This work provides an account of the culture and customs of the people of Britain, as well as the Roman interactions with them. Similarly, the Roman geographer Pomponius Mela wrote about Scandinavia in the first century C.E. He provided a description of the geography, people, and customs of the region, as well as a description of the Roman invasion of the area.

What about Muslims? One of the earliest and most extensive accounts of Muslim cultures and societies can be found in the كتاب المسالك والممالك (Book of Ways and Kingdoms), written by Ibn Khordadbeh in the ninth century C.E. In this work, Ibn Khordadbeh provides descriptions of several different Muslim societies, including those in Syria, Iraq, and Egypt. He also gives accounts of the different customs and political organization of the Muslim world at the time. He extends beyond Arabia, writing about Viking traders of Scandinavia and providing a unique perspective on Viking culture, customs, and technologies.

The writings of the Persian poet and historian Abulfeda (1273–1331 C.E.) also provide critical insight into the relationships between different Muslim cultures. In his work, كتاب المختصر في أخبار البشر (Concise History of Humanity), Abulfeda provides a detailed description of the political and cultural history of the Muslim world. He also gives accounts of the relations between different Muslim civilizations and their respective rulers and societies.

The Persian poet Saadi (1184–1283 C.E.) also wrote of the interactions between the East and West. In his poem “Gulistan”, he wrote of the strong relationship between the two regions, noting that the armies of the East were encouraged to conquer the West without “distressing its population” as well as speaking "nothing but good of the king."

Back to Europeans. In the 830s, a monk named Regino of Prüm wrote a chronicle of events in Europe, including accounts of the various Germanic tribes such as the Franks, Saxons, and Bavarians. Titling it the Chronicon, in it he noted several events of note, including that the Franks had defeated the Saxons in battle and that, on another day, the Franks attacked the invading Muslims in Aquitaine, killing 375,000.

The monk Widukind of Corvey (925 – ~973 C.E.) also wrote about the Franks and other tribes in Europe during the ninth century. Writing in the Res gestae Saxonicae (The Deeds of the Saxons), he noted that the Franks were a powerful people and that their kingdom was divided into provinces, each ruled by a nobleman. Widukind also wrote about the other Germanic tribes of the time, such as the Saxons, the Alemanni, and the Thuringii, describing their customs and beliefs.

The Spanish Christian traveler, Rodrigo Jiménez de Rada (1170–1247 C.E.), wrote extensively about his travels to France, Germany, England, and the Muslim world. In his De rebus Hispaniae (Of the affairs of Spain), de Rada provided a detailed description of the various peoples, their customs, and the differences between them. He noted the differences in language, dress, diet, and religion and commented on the religious wars that were occurring in Europe at the time.

The Franciscan friar, William of Rubruck (1220–1293 C.E.), traveled to the Mongol court of the Il-Khan in Persia in 1253. In his account of his travels, he discussed the various nationalities he encountered, including the Russians, the Poles, and the people of the Middle East. He described the different customs and beliefs of these peoples, as well as noting their use of different languages. His account is inconveniently titled Itinerarium fratris Willielmi de Rubruquis de ordine fratrum Minorum, Galli, Anno gratiae 1253 ad partes Orientales (Itinerary of brother William of Rubruck of the order of the Friars Minor, Gauls, in the year of grace 1253 to the Eastern parts).

The Italian explorer and geographer, Marco Polo (1254 – 1324 C.E.), is another well-known traveler who wrote about his travels in Europe and Asia. In his account of his travels, Polo described the people he encountered in many of the cities he visited, noting the differences between them in terms of clothing, customs, and beliefs. He also described the architecture, food, and trade of the various areas, as well as their political systems.

Lastly, but not least, the Venetian explorer and merchant, Niccolò de' Conti (1395 – 1469 C.E.), wrote extensively about his travels in India, China, and the Middle East. In his diaries and reports, Conti provided detailed descriptions of the different cultures he encountered and their beliefs, customs, and economic systems. He also discussed the way in which the different cultures interacted with each other, noting the importance of trade between them.

This is it for now. If you like other early accounts, I would be more than happy to provide more, especially Arab ones, including by Muslims, Christians, Jews, and even atheists.