If no date is given, use the abbreviation n.d. within parenthesis (= no date).

Example:

Johnson, A. (n.d.) The book. A publisher.

Several sources without year of publication

If you have several sources without year of publication, the sources are separated by adding a, b, c, etc. after n.d.
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.

In the reference, references with no date precede references with dates. 

Anderson, T. (n.d.)
Anderson, T. (2020)