If I were you I'll read from Book 1
https://www.mangago.zone/read-manga/ascendance_of_a_bookworm/
Since there are many skip parts, manga have more details (light novel have more content than manga and anime, give it a try as well) this manga is book 2 and almost finished.
The book 3 after anime season 3 is
https://www.mangago.zone/read-manga/ascendance_of_a_bookworm/
I mean parts 1-5 not book 1,2,3, etc.. Sorry for wrong words, if it confuses you a bit.
There are 5 parts of story, in manga format each parts have 1 book each with many volumes, same as light novel with each part have many volumes.
The series is currently split into five parts:
Part 1: Daughter of a Soldier (Volume 1 - Volume 3)
Part 2: Apprentice Shrine Maiden (Volume 4 - Volume 7)
Part 3: Adopted Daughter of an Archduke (Volume 8 - Volume 12)
Part 4: Founder of the Royal Academy's So-Called Library Committee (Volume 13 - Volume 21)
Part 5: Avatar of a Goddess (Volume 22 - Volume 33)
Hi. So I just finished the season 3 anime. Where should I start reading? Main got adopted by the lord