Do you test this solution or do you have an other track ?
Check Layout option of ThBook. One of the commands is "map-image": map-image <x> <y> <n/s/e/w/ne/nw/se/sw/center> <filename>
With help of this command you may place to final pdf any other pdf file - logo, picture of tree, bat, other map, …
1. generate the pdf file you want to "map-image"
2. generate the pdf file of second map and "map-image" first one.
You may do it in one compilation, but the order must be as above.
m.