A texture is basically just an image, such as a .png or .jpeg, which are themselves essentially a two-dimensional array of color data with whatever compression technology is being used to shrink that down; an engine typically imports these in such a way to make them more efficient for use in the game (as well as associating some metadata such as with Godot.) On modern technology these are rendered onto a quad and billboarded (as cards are geared towards processing in 3D.) A sprite sheet is a texture with lots of images; by extension a sprite displays a portion or all of this image using a texture atlas, which dictates what part of the sprite sheet is rendered at once. A tile is a portion of a sprite sheet used to draw a larger image, collected in a tile map, which can be orthographic (square), isometric (skewed), hexagonal, etc.
Honestly 2D graphics aren't the most complex topic, you'll pick it up as you make something. Also you don't need a book, you can get knowledge from plenty of sources, you will only gimp yourself relying exclusively on books. What's more important for making a game (particularly with an engine that already has this stuff figured out for you) is being able to produce something worth looking at, hence why I linked you an excellent resource on learning how to make 2D graphics. You can know everything there is to know about 2D graphics and it won't really matter if you can't produce them (unless you're making your own engine, but if that's the case why not check out Godot's source to see how 2D graphics work in there? Much more concrete real-world implementation than some generalized explanation.)
