As provided in the top answer to What books should I study to have a decent understanding of psychology?, I’ve compiled a reading list based on the books I acquired during my training as a therapist. The first section is a general introduction and they would answer your question on introductory material. I highly recommend reading these before going into the other 2 sections.
I have added the 2nd and 3rd sections for further reading. For a good understanding, I recommend you don't touch any of the books in the 3rd section unless you have a good grasp on the books in section 2.
I have often quoted from these books in my answers here.
General Introduction (Best read in order)
Sanders, P. (2011). First steps in counselling: A students' companion for introductory courses. Pccs Books.
Frankland, A., & Sanders, P. (1995). Next steps in counselling: a students' companion for certificate and counselling skills courses. PCCS books.
Reeves, A. (2022). An introduction to counselling and psychotherapy: From theory to practice. An Introduction to Counselling and Psychotherapy Sage. Contents and Chapter 1 Preview (PDF)
De Board, R. (2008). Counselling for toads: a psychological adventure. Routledge.
To be read after the general introductory for best understanding
Clarkson, P., & Cavicchia, S. (2013). Gestalt counselling in action. Sage.
Dryden, W., Jones, J., & Trower, P. (2015). Cognitive behavioural counselling in action. Cognitive Behavioural Counselling in Action. Sage
Jacobs, M. (2017). Psychodynamic counselling in action. Sage.
Mearns, D., Thorne, B., & McLeod, J. (2013). Person-centred counselling in action. Sage.
Rogers, C. (2012). Client Centred Therapy (New Ed). Hachette UK.
Further reading when you have a good grasp of the other 2 sections
Berne, E. (1968). Games people play: The psychology of human relationships. Penguin
Erickson, J. M., & Erickson, J. M. (1997). The life cycle completed: Extended version with new chapters on the ninth stage of development.
Harris, T. A. (2012). I'm OK, you're OK. Random House.
Jacobs, M. (2012). The Presenting Past: The Core Of Psychodynamic Counselling And Therapy: The core of psychodynamic counselling and therapy. McGraw-Hill Education (UK).
Sigelman, C. K., & Rider, E. A. (2021). Life-span human development. 10th ed. Cengage Learning.