Who does Holmes suspect is the master criminal in "The Red-Headed League"?

Holmes suspects the the person filling the position of Jabez Wilson's assistant of being the master criminal.  Holmes is very much aware of the details important to this crime, especially the assistant's role in ensuring that Mr. Wilson is away from the house at specific days and times.  The assistant, who is called Spaulding (actually John Clay), acts rather obviously as a coordinator of events, which indicates the has knowledge of elements that require that Wilson be positioned in ways typically unusual to him.