When your book has 8 or more authors, include the first six authors' names, then insert three ellipsis points (...), and add the last author's name.
Reference Example:
Crawley, R. B., Dockery, L. M., Branson, T. S., Carmichael, L. E., Carson, J. C., Findlay, A. F.,... Smith, D. M. (2015).
Manor houses of the early 1900s. London, England: Taylor & Francis.
The in text citation will be as any resource with six or more authors; that is, cite only the last name of the first author followed by et al. and the year. (p.175)
In-text Example:
Crawley, R. B. et al. (2015)