You can purchase a copy of Mark's debut novel and Audiobook at these outlets.
Audible - US - https://www.audible.com/pd/B0BJ35BK2L/?source_code=AUDFPWS0223189MWT-BK-ACX0-325673&ref=acx_bty_BK_ACX0_325673_rh_us
Barnes & Noble - https://www.barnesandnoble.com/w/six-bullets-mark-colt/1139091687?ean=9781666273779