The Structure of a Series: Holmes Omnibuses

After Holmes died, Doyle continued the series in two ways. First, some later written stories were set earlier, before the death. Secondly, other stories revealed that the death had been merely apparent. Thus, the series, in the order of publication, divides into three sections corresponding to three potential omnibus collections:

(I) two novels and two collections culminating in the death;
(II) two later written novels set before the death;
(II) three collections set after the return.

These sections are linked by the Moriarty organization:

(I) ends with the downfall of Moriarty;
(II) ends with Holmes planning the downfall of Moriarty whose chief of staff, we learn, is Sebastian Moran;
(III) begins with the arrest of Moran who had escaped after the downfall of Moriarty.

Thus, finally, the organization is destroyed. Other large, or at least important, organizations mentioned are:

the Scowrers, who engaged Moriarty to exact their revenge;
the Ku Klux Klan;
the Red-Headed League conspirators;
a German spy ring. 

However, the vast majority of Holmes' cases involve only local crimes or mysteries. Whereas James Bond later spent most of his time fighting either SMERSH or SPECTRE, Moriarty's organization appears only in the three works that link the three sections of the series. 

The Structure of a Series: Conan Doyle II

Let's run through that minimalist Holmes series again. The idea is to list only works presenting beginnings, turning points or endings, thus outlining the structure of the series, so that all other works are defined by their relationship to these key works:

before or after Watson's first meeting with Holmes;
before or after Watson's first marriage;
before or after Reichenbach;
before or after retirement etc.

We are concerned neither with publication order nor with the order of Watson's narration but solely with the chronological order of fictitious events.

Holmes recounts to Watson both his first case and the case that started his career:

"The Adventure of the Gloria Scott"
"The Adventure of the Musgrave Ritual"

Since these cases were pre-Watson, they must be the beginning of the series. Watson meets Holmes and moves into Baker St in an introductory novel, A Study in Scarlet, then marries and moves out of Baker St in the sequel, The Sign of Four. Thus, the Holmes series would have been complete in only two volumes had Doyle not then started the series of short stories, including the two already mentioned. This second series begins with "A Scandal in Bohemia" and ends with "His Last Bow." So far, the minimalist series comprises:

"The Adventure of the "Gloria Scott"
"The Adventure of the Musgrave Ritual"
A Study in Scarlet
The Sign of Four
"A Scandal in Bohemia"
"His Last Bow"

To these, we should add the middle (the "death" and the return):

"The Adventure of the 'Gloria Scott'",
"The Adventure of the Musgrave Ritual"
A Study in Scarlet
The Sign of Four
"A Scandal in Bohemia"
"The Adventure of the Final Problem"
"The Adventure of the Empty House"
"His Last Bow"

the two Mycroft stories:

"The Adventure of the 'Gloria Scott'"
"The Adventure of the Musgrave Ritual"
A Study in Scarlet
The Sign of Four
"A Scandal in Bohemia"
"The Adventure of the Greek Interpreter"
"The Adventure of the Final Problem"
"The Adventure of the Empty House"
"The Adventure of the Bruce-Partington Plans"
"His Last Bow"

one story announcing the retirement and one describing the retirement from which Holmes temporarily returns in "His Last Bow":

"The Adventure of the 'Gloria Scott'"
"The Adventure of the Musgrave Ritual"
A Study in Scarlet
The Sign of Four
"A Scandal in Bohemia"
"The Adventure of the Greek Interpreter"
"The Adventure of the Final Problem"
"The Adventure of the Empty House"
"The Adventure of the Bruce-Partington Plans"
"The Adventure of the Second Stain"
"The Adventure of the Lion's Main"
"His Last Bow"

two other stories that I have argued are significant enough to be included:

"The Adventure of the 'Gloria Scott'"
"The Adventure of the Musgrave Ritual"
A Study in Scarlet
The Sign of Four
"A Scandal in Bohemia"
"The Red-Headed League"
"The Adventure of the Greek Interpreter"
"The Adventure of the Naval Treaty"
"The Adventure of the Final Problem"
"The Adventure of the Empty House"
"The Adventure of the Bruce-Partington Plans"
"The Adventure of the Second Stain"
"The Adventure of the Lion's Main"
"His Last Bow"

The remaining two novels, written after the account of the "death" but before the account of the return, have to be regarded as occurring before the "death." This is explicit in the case of The Valley of Fear which retcons Moriarty. Thus, the complete minimalist series comprises, in this order:

"The Adventure of the Gloria Scott" (first case)
"The Adventure of the Musgrave Ritual" (the start of Holmes' career)
A Study in Scarlet (first meeting with Watson)
The Sign of Four (Watson's first marriage)
"A Scandal in Bohemia" (the first short story)
"The Red-Headed League" (an important pre-Moriarty case)
"The Adventure of the Greek Interpreter" (introduces Mycroft)
"The Adventure of the Naval Treaty" (an important international case)
The Hound of the Baskervilles (the most famous and most filmed novel)
The Valley of Fear (retcons Moriarty)
"The Adventure of the Final Problem" ("death")
"The Adventure of the Empty House" (return)
"The Adventure of the Bruce-Partington Plans" (explains Mycroft's job)
"The Adventure of the Second Stain" (announces the retirement)
"The Adventure of the Lion's Mane" (describes the retirement)
"His Last Bow" (temporary return from retirement)

The Structure of a Series: Conan Doyle

Of the Sherlock Holmes short stories, what is the minimum number necessary to outline Holmes' career? 

"A Scandal in Bohemia" and "His Last Bow" are the beginning and the end. The latter appropriately refers back to the former.

"The Final Problem" and "The Empty House" are the middle: the "death" and return.

"The Greek Interpreter," set before the "death," introduces Mycroft Holmes. "The Bruce-Partington Plans," set after the return, explains Mycroft's role in the Government. 

"His Last Bow" is a return from the retirement that is announced in "The Second Stain" and described in "The Lion's Mane."

Therefore, I think these eight are the minimum. "The Bruce-Partington Plans" and "The Second Stain" name certain foreign agents working in London. This links these stories directly to each other and indirectly to "His Last Bow" in which Holmes and Watson apprehend Von Bork who is as significant in international espionage as Moriarty was in national crime.

I would also include "The Red-headed League" and "The Naval Treaty." John Clay, who is behind the Red-headed League hoax, is high in London crime which means that Holmes must be well on the road to identifying Moriarty. At least two dramatizations have retroactively linked Moriarty to the Red-headed League. Holmes says that Clay is "the fourth smartest man in London." The three smartest must be Moriarty, Mycroft and Sherlock although Doyle was probably not thinking that when he wrote this story. 

"The Naval Treaty," coming between "The Greek Interpreter" and "The Final Problem," is a third story about an important stolen document and thus fits in with "The Bruce-Partington Plans" and "The Second Stain."

If we include the novels, then we must include them all. Holmes and Watson meet in A Study in Scarlet. Watson marries in The Sign of Four, thus explaining why he is not living at Baker Street in many of the stories starting with "A Scandal in Bohemia." The Valley of Fear, set before "The Final Problem," features a criminal gang who, although they are brought to justice, engage Moriarty to exact their revenge. Finally, The Hound of the Baskervilles, perhaps the novel that has been most often filmed, cannot be left out of any list of Holmes stories.