Susan Cooper: The Dark Is Rising Series

This is not a typical "series." It's a five-book sequence of fantasy novels written for young people.
Main Characters: Merriman Lyon, Will Stanton
Setting: England
Bold type indicates a book I've read.

Over Sea, Under Stone (1965) [See Review]
The Dark Is Rising (1973) [See Review]
Greenwitch (1974) [See Review]
The Grey King (1975) [See Review]
Silver on the Tree (1977)

Maud Hart Lovelace: The Betsy-Tacy Books

Main Characters: Betsy Ray and her best friends for life, Tacy and Tib
Setting: Mainly Hill Street, although the later books branch out a bit
Bold type indicates a book I've read.

Betsy-Tacy (1940) [See Review]
Betsy-Tacy and Tib (1941)
Betsy and Tacy Go Over the Big Hill (1942)
Betsy and Tacy Go Downtown (1943)
Heaven to Betsy (1945)
Betsy in Spite of Herself (1946)
Betsy Was a Junior (1947)
Betsy and Joe (1948)
Betsy and the Great World (1952)
Betsy's Wedding (1955)

Arthur C. Clarke: The Space Odyssey Series

This is a series of science fiction stories and novels that take place over a fairly long period of time and in different parts of the universe. In general, the main characters and settings differ in each of the works.
Bold type indicates a book I've read.

"The Sentinel" (written in 1948, first published in 1951 as "Sentinel of Eternity")
"Encounter at Dawn" (first published in 1953)
2001: A Space Odyssey (1968)
2010: Odyssey Two (1982)
2061: Odyssey Three (1987)
3001: The Final Odyssey (1997)

Frank Herbert: The Dune Saga

Main Characters: Duke Leto Atreides, head of House Atreides; Lady Jessica, concubine of the Duke, mother of Paul and Alia; Paul Atreides, the Duke's only surviving son; Alia Atreides, Paul's younger sister; many, many others
Setting: Mainly the planet Arrakis
Bold type indicates a book I've read.

Dune (1965)
Dune Messiah (1970)
Children of Dune (1976)
God Emperor of Dune (1981)
Heretics of Dune (1984)
Chapterhouse: Dune (1985)

Raymond Chandler: Philip Marlowe Mysteries

Main Character: Private detective Philip Marlowe
Setting: California, mainly Los Angeles
Bold type indicates a book I've read.

The Big Sleep (1939)
Farewell, My Lovely (1940)
The High Window (1942)
The Lady in the Lake (1943)
The Little Sister (1949)
The Long Goodbye (1954) (Edgar Award for Best Novel, 1955)
Playback (1958)
Poodle Springs (1959) (incomplete; completed by Robert B. Parker in 1989)

Josephine Tey: Inspector Alan Grant Mysteries

Josephine Tey is the pseudonym of Elizabeth MacKintosh, who also published under the name of Gordon Daviot.
Main Character: Inspector Alan Grant of Scotland Yard
Setting: England and Scotland
Bold type indicates a book I've read.

The Man in the Queue (1929), writing as Gordon Daviot
A Shilling for Candles (1936)
To Love and Be Wise (1950)
The Daughter of Time (1951)
The Singing Sands (1952)

Agatha Christie: The Miss Marple Mysteries

Main Character: Miss Jane Marple, elderly spinster and amateur sleuth
Location: St. Mary Mead and other locales, mostly in England
Bold type indicates a book I've read.

Murder at the Vicarage (1930)
The Tuesday Club Murders (1932), aka: The Thirteen Problems
The Regatta Mystery (1939)
The Body in the Library (1942)
The Moving Finger (1943)
Mousetrap & Other Stories (1950)
A Murder is Announced (1950)
Murder with Mirrors (1952), aka: They Do It With Mirrors
A Pocket Full of Rye (1953)
What Mrs. McGillicuddy Saw (1957), aka: 4:50 from Paddington
The Adventure of the Christmas Pudding (1960)
Double Sin & Other Stories (1961)
The Mirror Crack'd (1962), aka: The Mirror Crack'd From Side to Side
A Caribbean Mystery (1964)
At Bertram's Hotel (1965)
Nemesis (1971)
Sleeping Murder (1976)