Sword Art Online Light Novels and Manga guide for Newcomers.
Manga

Light Novel, ? vol, 2009
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Main series, split into X Arcs:
Aincrad - Volume 01-02
Fairy Dance - Volume 03-04
Phantom Bullet - Volume 05-06
Mother's Rosario - Volume 07
Early and Late - Volume 08 (Side-stories)
Alicization - Volume 09-18
Moon Cradle - Volume 19-20
Kiss and Fly - Volume 22 (Side-stories)
Unital Ring - Volume 21-??
Aincrad - Volume 01-02
Fairy Dance - Volume 03-04
Phantom Bullet - Volume 05-06
Mother's Rosario - Volume 07
Early and Late - Volume 08 (Side-stories)
Alicization - Volume 09-18
Moon Cradle - Volume 19-20
Kiss and Fly - Volume 22 (Side-stories)
Unital Ring - Volume 21-??

Light Novel, ? vol, 2012
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Secondary saga set during Aincrad.

Light Novel, ? vol, 2014
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Spin-off set in the Gun Gale Online VRMMORPG. Starts after Mother's Rosario and spans during the events of the main saga.

Light Novel, 3 vol, 2016
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Spin-off set in the Asuka Empire VRMMORPG. Starts after Mother's Rosario.

Manga, ? vol, 2021
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New manga version of Aincrad, adapting events from various sources.

Manga, 2 vol, 2018
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Manga version of Progressive Part 2.

Manga, 3 vol, 2020
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Manga version of Progressive Part 3.

Manga, 2 vol, 2021
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Manga version of Progressive Part 4. Cancelled.

Manga, 3 vol, 2014
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Manga version of Mother's Rosario.

Manga, 4 vol, 2015
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Manga version of SAO Alternative: Gun Gale Online. Adapts Volume 01 and part of Volume 02.

Manga, 8 vol, 2013
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Spin-off set in VRMMORPG Alfheim Online after Mother's Rosario.

Manga, 5 vol, 2017
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Manga version of Sword Art Online - The Movie: Ordinal Scale.

Manga, 5 vol, 2016
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Manga version of Alicization.