This is a book borne of experience rather than theory. What Irvin D Yalom does here is offer counsellors and psychotherapists the benefit of his own discoveries about what works. While he does make occasional reference to various schools of thought (e.g. neo-Freudian) it is clear that his interest is in getting 'alongside' people rather than in analysing or diagnosing them. Indeed, he cautions against diagnoses and advises therapists to view those who seek their help as 'fellow travellers' (rather than as afflicted clients).
Not being a therapist myself, I am not able to comment from that particular perspective, but it seems to me that what Yalom is writing about is the kind of pastoral care that, in the past, many people may have sought from their priests, ministers or rabbis. It is interesting that Yalom dislikes the term 'client' (which suggests a business relationship), preferring instead to use the term 'patient' (which seems rather more clinical) - but 'parishioner' might almost be a more fitting expression (though, presumably, there cannot be too great a 'flock' of 'parishioners' at any one time!). I make this comment because Yalom appears, essentially, to be urging therapists to act as secular clergy, supporting and empathising with individuals and, on occasions, giving them direction.
Given his relational/pastoral emphasis, it is unsurprising that he is dismissive of particular fixed approaches to therapy. He is certainly critical of the Person-Centred Approach pioneered by Carl Rogers, but he reserves some of his most scathing remarks for formulaic 'evidence-based therapies' such as CBT (Cognitive Behavioural Therapy).
The Gift of Therapy is actually an enjoyable as well as informative read, precisely because it stresses relationship with the human subject rather than adherence to a method.