Reddit reviews Life: The Science of Biology

"life"by Sadava et. al. is a good gen bio text, with a decent amount of material on plant biology. Im sure you can find it online as a digital copy somewhere.
Your questions are asking a lot more that I think you realize. to be general plant cells are surrounded by a cell wall that fills the spaces in between cells. Some cells form junctions with neighboring cells and essentially wash out all of their internals to become the veins of the plant, they are supported by their neighboring cells biochemical processes.

Virus can directly enter the cells of the plant, and utilize the cells own machinery to reproduces itself, and then be released into neighboring cells. The virus usually attaches via specific protein structures on its surface, to either receptors on the target cell, or in some cases, directly through the membrane. Sadave gets into some immunology but I dont think too much detail about plant immunochemistry.