Plotinus

From the preface: The Enneads, from the great philosopher Plotinus, has exerted a considerable influence on Western civilization, including our present time.
Plotinus himself was taught by Ammonius Saccas, the "theodidactos", or God-taught philosopher in Alexandria who brought several philosophical systems together in his Neo-Platonic School.

Those of this School were known as the "Philalethians" (lovers of the truth); while others named them the "Analogists," on account of 'their method of interpreting all sacred legends, symbolical myths and mysteries, by a rule of analogy or correspondence' as indeed one can find in the writings of Porphyry.

"Ammonius' chief object ... was to extract from the various religious teachings, as from a many-chorded instrument, one full and harmonious melody, which would find response in every truth-loving heart". The central idea of his eclectic theosophical school was that of a "single Supreme Essence, Unknown and Unknowable".

Much of what Plotinus writes undoubtedly has its source in the above mentioned School. He was also influenced by the Stoics, and other philosophers. (quotes from a linked article in the preface).

In his Enneads he treats of the following topics:

First Ennead: Human or ethical topics;
Second and Third Enneads: cosmological subjects or physical reality;
Fourth Ennead: about the Soul;
Fifth Ennead: knowledge and intelligible reality;
Sixth Ennead: Being and what is above it, the One or first principle of all.

Plotinus: Enneads
Complete book!