Roms (and saves) can be easily be placed on an external disc - https://retropie.org.uk/docs/Running-ROMs-from-a-USB-drive/.

For configurations (almost all of them are kept in /opt/retropie/configs), there's no out-of-the-box solution available, since they require a Linux filesystem and generally users have a PC/macOS compatible partitioning schema on their external disc.

If you want to dedicate a SSD just for running RetroPie, you can just install RetroPie on the SSD drive (same process as installing it on a sdcard) directly.

Has anybody ever came across a good learning book for this stuff?

RetroPie specific documentation - https://retropie.org.uk/docs/. Raspberry Pi documentation - https://www.raspberrypi.com/documentation/ (has a Getting started section and a Using RasPI OS guide). The RetroPie image is based on RasPI OS. For Linux in general, I'd recommend the Debian user guide (https://www.debian.org/doc/manuals/debian-reference/index.en.html). RasPI OS is based on Debian, so it should apply to a RetroPie installation.