To give you some more specific answers, could you tell me:
* What genre are you interested in? (Fantasy, science fiction, mystery, romance, etc.)
* What kind of series are you looking for? (Long-running series, short series, completed series, ongoing series, etc.)
* Do you have any specific authors or titles in mind?
For example, here are some popular series in different genres:
* Fantasy: The Wheel of Time by Robert Jordan, A Song of Ice and Fire by George R.R. Martin, The Kingkiller Chronicle by Patrick Rothfuss
* Science Fiction: The Expanse by James S.A. Corey, Dune by Frank Herbert, The Culture series by Iain M. Banks
* Mystery: The Sherlock Holmes series by Arthur Conan Doyle, The Poirot series by Agatha Christie, Harry Bosch series by Michael Connelly
* Romance: Outlander by Diana Gabaldon, The Bridgerton series by Julia Quinn, Twilight by Stephenie Meyer
I can give you more specific recommendations based on your preferences!