I don't know if is just a bug in the BQ Cervantes device driver and the lpath is always relative to where metadata is stored. It seems to be the case in Kindle devices. See
https://github.com/koreader/koreader...ment-640260257
In that case I would assume that I can always get the book full path knowing the root of the library + book lpath. A confirmation would be nice, specially for Android & pocketbook devices.