What are the Hermetic Writings?
The specific body of literature referred to by the term "Hermetic writings" is, like most things having to do with Hermeticism, hard to define concisely. Much like the nature of Hermes Trismegistus, there will be different answers from different sources. For example, in attempting to define something as simple as the number of "Hermetic books", opinions include (Stock, 626-627)

• 36,525 books, a number assigned by an Egyptian priest and having significance in relation to the the duration of the dynasties of Egypt, or
• 20,000 books, according to a Roman historian, or
• 42 "essential books" according to Clement of Alexandria, or
• Simply "many books" according to another Roman historian.

The exact number is not known, but there is a general consensus on the structure of the Hermetic literature. The Hermetic writings can be divided into two main sections: writings on sciences, especially those of occult significance, and writings on philosophy, presenting the Hermetic doctrine and view of the world (Mahé, 288). While the scientific texts are numerous and cover a broad range of topics - astrology, astronomy, alchemy, magic, geography, topography, education, worship, laws, priestly conduct, anatomy, disease, organs of the body, medicine, opthalmology, and even gynecology (Stock, 626-7; Mahé 288), just to name a few - it is the philosophical texts which are of most interest in studying Hermeticism, so I will be focusing primarily on those texts.

The Essential Hermes
Among the philosophical texts, there are six main groups (Mahé, 288):

1. The Fragmenta Hermetica, which is quite literally fragments of Hermetic writings quoted by other authors over a wide period of time.
2. The Asclepius, also called the "Perfect Sermon" (Greer, "An Introduction to the Corpus Hermeticum"), from roughly the second or third century CE.
3. The "Nag Hammadi codex", a set of Coptic books containing three Hermetic works, from around 350 CE.
4. The Stobaei Hermetica, more fragments, compiled around 500 CE.
5. The Definitions of Hermes Trismegistus for Asclepius, late sixth century CE.
6. The Corpus Hermeticum, fifteen books of Hermes, probably from the second or third century CE..

While all of these works are dated from around the second century CE and later, the origins of the texts - both philosophical and scientific - are much older, and the known Hermetic writings represent the surviving remnants of a larger literature (Mahé, 289; Martin, 146; Greer, "An Introduction to the Corpus Hermeticum"), with the general assumption in the ancient world that the Hermetic body of work was based on ancient knowledge of Egypt, widely regarded as the storehouse of antique wisdom.
And not all Hermetic writings are equal. Some scholars assert that the Corpus Hermeticum and the Asclepius, despite their later composition, represent "foundational" writings of Hermeticism (Greer, "An Introduction to the Corpus Hermeticum"), and those works are among those most often referred to in the general study of Hermeticism. Some sources take this even further, and refer to a section of the Corpus Hermeticum, the book titled Poemander ("Man-Shepherd" or "Shepherd of Men"), as the essential example of Hermeticism (Martin, 146).