So I typically run my games on Roll20, and in the past I purchased a particularly fantastic set of Terrains from Gabriel Pickard that I generally use for most occasions. I do recommend his work. Usually his stuff is sold in packs, and include things like random, theme appropriate objects to place on the terrain, and different mashup tools to help sell the illusion of a new battlefield each time your players come to the table. I'll see about structuring the maps in a similar fashion. That way a limited set of images can be reused and abused for a wide variety of purposes instead of just one instance of one combat.
On the note of going crazy trying to create maps for every scenario. You don't need to! A wise GM once told me that the best way to prep is to only prep what you need for the next session and nothing else. Treat this book the same way. Don't prep everything from the start all the way to the finish, odds are you won't get that far with most playgroups. Prep maybe a combat or two in advance, refresh yourself on the upcoming content, and let the rest play out. Maybe that's not your style, and that's fine, but it's what has worked for me. In the meantime I'll be working on those battlemaps.