How come they appear in subsequent novels, if they're dead?

They appear together in four full-length novels and one collection of short stories.

Spinalonga has appeared in novels, television series, and a short film.

He then went on to appear in later novels.

Renko has since appeared in six other novels by Smith.

He appeared in six novels by Marquand published between 1935 and 1957.

Some of the characters appear in both novels with the same occupation and characteristics.

He appears in four novels, and is mentioned in at least one more.

He appeared in more than twenty novels, all of them became a bestsellers.

Both characters also appear in many non-canonical novels and stories.