|DITA map files / The bookmap feature|
For complex collection structures, only use bookmap at the top level. Any nested maps should be ditamaps.
You should not nest bookmaps within ditamaps, as the structure will cause confusion for both processing software and for documentation authors.
If you need to re-use content from a bookmap inside a ditamap, then a lower-level ditamap should be nested in the bookmap, and that lower level ditamap re-used in the main ditamap. In fact, except for small documents, you should aim to have the top-level bookmap contain only map references, and no topic references.