What is commonly referred to as "esotericism" in the context around Biodynamics is really just seeing behind the veil of material appearances. The only thing that makes the insights of biodynamics obscure or "esoteric" is that they aren't available on the surface of things.
There many hidden processes in the soil, so many that scientists often refer to the dynamics of soil as a sort of "black box" -- you add something to the hidden complexity of soil life and you get surprising results that often defy materialistic explanations. But we know that most of our lives are our experiences of the world: how I feel and what I think about the world is a private experience, and yet it is almost all I encounter. How can we step outside our own subjective experience?
To each other person we are like a hidden world, and yet we know how much goes on inside ourselves! In every relationship between two people, we only see the "tip of the iceberg" with our eyes, but we know there is much more going on below the surface.
Esotericism aims at applying pattern recognition to the invisible realms of life, feeling, and thought. This process is based on the assumption that the universe is not lifeless, is not without feeling, and is not thoughtless. If it were, how could it produce life, feeling, or thought? Biodynamics operates on the hopeful hypothesis that the world tends towards order and that life begets life, and that we are not in a completely senseless cosmos.
As such, the worldview that asserts that everything is mere chance and that we are merely an amalgam of random atoms does not tend to know how to think about these invisible, but essential, qualities of life, feeling, and thought -- all of which tend to be explained away as "chemical reactions" and yet no one has been able to identify a chemical reaction that represents, for example, the pure experience of love or the beauty of a rose or the thought of justice.
Biodynamics merely asserts that love is real, and that a rose is real, and that ideas are real, not merely a mirage of a material universe looking back on itself. Accepting that, the investigations in to obscure corners of the soul (and the soil) lead us into many unusual possibilities, including the compost preparations. These remedies are not meant to be obscure anymore than the experience of beauty or love is meant to be obscure -- it is only obscure if we have not experienced that they work. As such, Biodynamics is not an abstract philosophy but is drawn from the real world and its inner workings. Biodynamics is chiefly about what works and what can be in service of LIFE.
The good thing is you don't have to be an esotericist to utilize what is fruitful from esotericism. But if you wish to explore it, some of the fundamental introductory texts to the esotericism behind Biodynamics include:
Intuitive Thinking as a Spiritual Path by Rudolf Steiner
Meditative Poetry of Rudolf Steiner
Theosophy by Rudolf Steiner
An Outline of Esoteric Science (sequel to Theosophy)
What is Biodynamics? by Steiner (introduction by Hugh Courtney, presuming familiarity with Theosophy and Esoteric Science before reading)