I am interested in learning about $G_2$ manifolds and am aware that one of the canonical references is Joyce's Compact Manifolds with Special Holonomy. I am certain that my background is, at this moment, not enough to read this research monograph so I would like to know if there are books that can bring me up to speed. My background is in Lee and Do Carmo level.
I started with this 58 page set of notes by Joyce https://arxiv.org/pdf/math/0108088.pdf, it was published as the first section of a three section book "Calabi-Yau Manifolds and Related Geometries" (the other two sections were written by Gross and Huybrechts). This is lighter than his textbooks.
I also recommend this introductory talk of Robert Bryant https://www.youtube.com/watch?v=j0qBv62pNWw.