If the same author(s) has several publications in the reference list, with the same year, they are sorted alphabetically by their titles. To be able to distinguish them when you cite them in the text, where you only use author and year, you add a letter after the year as well. Add a, b, and c etc. in the order they appear in the reference list:

• Phillips, H., & Anderson, T. (2018a). Algebra: an introduction. Cambridge University Press.
• Phillips, H., & Anderson, T. (2018b). Mathematical problems for everyone. Cambridge University Press.
• Phillips, H., & Anderson, T. (2018c). String theory explained. Cambridge University Press.

This also applies if the sources have no year. Please note that there is a hyphen between n.d. and the letter:

• Anderson, T. (n.d.-a). Algebra from the past. Cambridge University Press.
• Anderson, T. (n.d.-b). Mathematical journeys through time. Cambridge University Press