ISTJ = repository of information.
Holmes hold a lot of facts and figures in his head. Now whilst an INTP in their main area of interest is an expert I've yet to meet one who had such complete knowledge of the minutia of their subject and certainly not when the subject was so broad as Holmes' was. Also his reference to information is specific and not brushed over at all which is more S than N.
I was going to put an argument down for why Holmes should be an intuitive person and not a sensor but I find that when I look to Holmes' style and his fascinations it does tend to come over all S. I guess that would make Watson an ENFJ.... a theory told to me long ago that Holmes and Watson were a successful duo because they were opposites.
Oh hang on a minute.
Holmes is an intuitive person. When he looks at evidence he does not see it's qualities as an object or a specific instance, he sees it as what it means in reference to this case. Holmes is always in context and therefore intuitive. If he took pieces of information and considered them outside of the chain of evidence until it was proven that they should be included then he would be a sensor.... I think.
Anyhow reading Holmes is like stepping into my own mind a lot of the time. Ergo I consider him an INTP despite the ISTJ tendancies.
I don't see the whole ISTP though. I can see it from a clinical function/ process type of view but not as a holistic type.