You cannot convert whole InDesign books (.indb) to a single IDML file, the program doesn't support this function. At best, you can convert (export) individual files from .indb files and save each of them in a separate .idml file. This can of course only be done using InDesign itself, as no other software is capable of this conversion.
Convert Adobe InDesign publication to InDesign Markup Language document.
We found 2 software records in the database related to .indb to .idml file format conversion.
Microsoft Windows software - convert indb to idml on Windows
Professional desktop document publishing software from Adobe
Convert from indb file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned indb to idml. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert indb to idml on OS X
Mac version od Adobe's popular document designing and publishing software