Can a subdomain also have a hostname?

A computer has a hostname and an IP address.

A subdomain is a collection of computers and IP addresses that share a common IP-address range and second- (or more) level domain name.

Your question is somewhat like asking "How do I get the roots off an apple?" An apple doesn't have roots. An apple sits on a tree. The tree has roots.

As stated above, domain name hierarchy goes from right to left.

subdomain.domain.com (i.e. book.quality.com - book is a subdomain of quality. Quality is a subdomain of com). Com is called the root zone.

When it comes to internet naming conventions these are all tied to DNS zones. Each domain can contain multiple records within a DNS zone that point to various IP addresses. So, you can add records for domain.com and you can add records for subdomain.domain.com.

If you want book.quality.com to be another sub-domain and be capable of having it's own A, MX, PTR, and other records then you will create a new zone in DNS. If book.quality.com simply points to a specific server then you will just create an A record for it in the quality.com zone. If the latter, then book would be a host entry in the DNS zone quality.com.

Despite the technicalities, a short answer to your question is that if you own quality.com you can create as many sub-domains and DNS zones as you want behind that. i.e. a.quality.com; b.quality.com; c.b.quality.com; more.c.b.quality.com