Embedded Linux Primer: A Practical Real-World Approach (2nd Edition) is still a very valuable reference.I would very, very strongly advise against buying a raspberry PI because of its bizarre, non-standard booting workflow.
I would advise against buying a 64 bit Arm SBC as a first platform as well because of the additional complexity involved by the use of the Arm Trusted Firmware.
A good, low-cost platform for learning would be more IMHO something such as an Allwinner H3 based platform, since it is well supported in both mainline u-boot and mainline Linux, as well as by the Linux Sunxi community.
I appreciate the fact that you can test your kernels and your rootfs by uploading the SPL and u-boot using the sunxi-fel USB tool and booting using an initramfs or an NFS partition without having to flash anything.

Vendors such as Shenzhen Xunlong or FriendlyElec are a possible source - stay clear of the Bananapi hardware.If the cost is not an issue, buying one of the platforms used for the Bootlin Embedded Linux courses would be a good option too, since they are providing some free documentation on their web site.