"Who am I?" book is a collection of questions and answers that occurred between Guru and disciple. These questions mainly dig into the "Self-enquiry" and bothering questions like "Who am i"?.
There is not much difference between Shankaracharya and Raman Maharshi's teachings. Raman Mahrarsi's teaching birthplace is Adavitha philosophical system. Discrimination and inquiry are the tools used to remove unreal simultaneously.
According to Raman Maharsi, To find"Who am I?", We need to negate everything unreal so only the real can remain. The final awareness answers the question. The nature of this awareness is constant bliss, When the world is negated as unreal and illusion, Awareness alone exists in our perception.
To discover the awareness clouded by ignorance, First, we need to silence the mind. Silencing the mind is easier when we know its nature. Mind alone doesn't exist, It is just a collection of thoughts derived from our everyday action and memory. Mind is residing power in the self, When the mind comes out of the Self, the world appears. Therefore, when the world appears (to be real), the Self does not appear; and when the Self appears (shines) the world does not appear. When one persistently inquires into the nature of the mind, the mind will end leaving the Self (as the residue).
Once you inquire about illusory mind, Dive deep into the core of "i" by questioning "To whom it arise?". Through this inquiry, We will reach inward and the root of awareness.
Maharshi mainly emphasizes "self-inquiry" to discover the nature of the self free of "I" thought. and he also emphasizes breathing which is the gross form of Mind. Through constant self-inquiry and control of breathing, we can discover real awareness.
However, Maharsi argues inquiry is superior to meditation in the path of realization. because inquiry consists of retaining the mind on the self, meditation is a contemplation on Brahman. If you are familiar with the "Advaith" philosophical system, it's easier to understand the philosophy of Maharsi.