Year Bestsellers for 2020:
This is the list of the all volumes with total sales of 10k+ from the weekly rankings in calendar year 2019 (data commences 30th December 2019). Data from the monthly (lol remember them?), half yearly and yearly rankings are not included. Sales that cannot be confirmed as taking place during this year are not included in the data.
Titles in italics came out in a previous year
The data consists of the data for bunko, books, literature and light novel rankings.
1 323224
Kimetsu no Yaiba Shiawase no Hana
2 322703
Kimetsu no Yaiba Kataha no Chou
3 172309 Overlord 14
4 95848 Youkoso Jitsuryoku Shijou Shugi no Kyoushitsu e 2-nensei hen 1
5 71622 Kusuriya no Hitorigoto 9
6 70383 Seishun Buta Yarou 10
7 53380 Shousetsu Eiga Doraemon Nobita no Shin Kyouryuu
8 50489 Ryuuou no Oshigoto! 12
9 43765 Mahouka Koukou no Rettousei Shiba Tatsuya Ansatsu Keikaku 3
10 42135
Kono Subarashii Sekai ni Shukufuku wo! Yorimichi!
11 35626 Souyaku Toaru Majutsu no Index 1
12 35490 Fate/strange Fake 6
13 32550
Sword Art Online 23
14 27000 Tondemo Skill de Isekai Hourou Meshi 8
15 26924
Hakugin no Oka, Kuro no Tsuki (Juuni Kokuki) 4
16 26848
Hakugin no Oka, Kuro no Tsuki (Juuni Kokuki) 3
17 25621 Goblin Slayer 12
18 24529 Wotaku ni Koi wa Muzukashii Shousetsu
19 24448 Osananajimi ga Zettai ni Makenai Love Come 3
20 23508
Hakugin no Oka, Kuro no Tsuki (Juuni Kokuki) 1
21 23007 Youjo Senki 12
22 22784 Death March kara Hajimaru Isekai Kyousoukyoku 19
23 22685
Kokuhaku Yokou Renshuu Heroine Ikusei Keikaku
24 22597 Kumo desu ga, Nani ka? 12
25 22280 Yotsugo Gurashi 5 part 2
26 21959 Strike the Blood 21
27 21674
Jujutsu Kaisen Yoake no Ibaradou
28 21387
Hakugin no Oka, Kuro no Tsuki (Juuni Kokuki) 2
29 20649 Rokudenashi Majutsu Koushi to Akashic Records 16
30 20598 Shin High School DxD 4
31 20413 Date A Live 22
32 20346 Honzuki no Gekokujou 5 part 1
33 20266 Itai no wa Iya nano de Bougyoryoku ni Kyokufuri Shitai to Omoimasu. 9
34 19294 Kami-tachi ni Hirowareta Otoko 8
35 19014 Yotsugo Gurashi 5 part 1
36 18204 Akasuka Oniyame Nikki 8
37 17896 Yahari Ore no Seishun Love Come wa Machigatteiru. Anthology 1
38 17667 Imouto Sae Ireba Ii. 14
39 17379 Hoshi no Kirby Mugen no Haguruma wo Sagase!
40 16830 Yahari Ore no Seishun Love Come wa Machigatteiru. Anthology 2
41 16319 Tantei Team KZ Jiken Note Yabai Shinyuu wa Shitteiru
42 15615 Seijo no Maryoku wa Bannou desu 5
43 13648 Nanatsu no Maken ga Shihaisuru 5
44 12834 Maou Gakuin no Futekigousha 6
45 12755
Kono Subarashii Sekai ni Shukufuku wo! Extra 6
46 12036 Magical Explorer 2
47 11946
Boku no Hero Academia The Movie Heroes: Rising
48 11603 Watashi no Shiawase na Kekkon 3
49 11492 Okashi na Tensei 4
50 10442 Yukiusagi no Oshinagaki Kazabana Mau Hi no Mizore Nabe
51 10342 Fairy Tale Chronicle ~Kuuki Yomanai Isekai Life~ 20
52 10148 Watashi, Nouryoku wa Heikinchi de tte Itta yo ne! 13
53 10093 Madou Gushi Dahliya wa Utsumukanai 4
(source: Oricon via refugee-chan)
2019 is here 
