And I suggest you delve into
Advaita Vedanta.
Shri Atmananda's works are excellent and I highly recommend them, but Adi Shankara's commentaries on the Upanishads are the gold standard.
In short, there are two key concepts:
Brahman
The Supreme Cosmic Principle or
Brahman is the One without a second, the whole and the only reality. Other than
Brahman, everything else, including the universe, material objects and individuals are unreal.
Brahman is the omnipresent, omnipotent, impersonal, transcendent reality of all Being. It is the substratum of the material world, which in turn is its illusory holographic self-manifestation. Due to ignorance or
avidya,
Brahman is visible as the material world and its objects; Brahman is beyond attributes and forms, which is why frequently its is spoken of by the Sanskrit phrase, "
neti, neti", or "not this, not this". It is often spoken of as "Satchitananda" (Sat is Truth, Chit is Consciousness and Ananda is Bliss).
Atman
The individual self (
Atman), is the
same as
Brahman. It is not a part of
Brahman that ultimately dissolves into or merges with
Brahman, but
Brahman itself . The "soul" (note the quotes; soul is a poor terminology) is
not an individual entity.
Atman is only one and unique. It is a false concept that there are several
Atmans or "souls".
Adi Shankara famously illustrated with the metaphor that just as the one ocean appears on the surface as many individual waves, so does the one
Atman (
Brahman) appears as multiple
Atmans in our bodies because of
maya, or false perception of Reality. When the
Atman is enveloped by
avidya (ignorance),
Atman becomes jiva, or a living incarnation with a body. However, this is not real; it is trapped in the matrix of
maya, or what is unreal. At the transcendental level, only the One without a second is real.
The only way to realize the above is through deep meditation or
Self-Enquiry.