Oh, dear, Bonnie.
The big question, particularly with the books, is Lady Helen.
This is a spoiler, sorry, but:
I divide the Lynley stories into two areas:
Before Helen and After Helen.
Lady Helen does wonders for Inspector Lynley, and then ...
he has to learn how to live without her.

It's safe to say -- granted I last read one of these books years ago --
that Lady Helen is in the Lynley series from the very very beginning.
More to the point,
since you mention Isabelle Ardery,
it is only with the loss of Lady Helen
that Isabelle Ardery's character eventually arrives.   

What happens to Lady Helen polarized Elizabeth George readers everywhere,
and some of them said they would never forgive EG for it.