I really love it when mangaka include essay afterwords or just little comments about their process at the end of volumes, especially if it's their own relationship with the topic that they're writing about, or even a brief discussion of their love for manga and comics.
One that I really liked is the essay included in the second chapter of Jiro Taniguchi's anthology "Izanaumono" where he talked about his love for European comics and the French comic artist Moebius.
A more popular work that had essays I really enjoyed reading was Shuzo Oshim's "Welcome Back, Alice" where he discussed his relationship with gender and sexual identity.
Another one I like was in Ching Nakamura's "Gunjou" where she gives us a playlist of songs she listened to while writing!!