Convert book settings to the new configuration system
The book settings at :
admin/structure/block/manage/book/navigation/configure aka hook_block_configure
Create a default config file and add it to the module.
Convert the admin UI in core/modules/book/book.admin.inc core/modules/book/book.module to read to/write from the appropriate config.
Convert any book code that currently accesses the variables used to use the config system.
Write an upgrade path from D7 -> D8 to migrate existing data to the new system and drop the appropriate variables.
This would be a good task for someone wanting to get up to speed on how the new config system works. That would be me, chriscalip.