I’ve been following the books "Agile Web Development with Rails " since Rails 3 version. Sure, I’ll never be grateful enough to the authors for the efforts they brought to publish all these books. But as for its latest version, I can, unfortunately, notice the decreasing quality of the material:

• there is no mention of how the source code of the ‘depot’ application has to be used. Instead of naming the folders according to the chapter, a weird naming was chosen: depot_a…depot_xb which provide no guess to which chapter it is related.
• some tests are failing because of incoherence of the provided material.

Rails 8 bringing a lot of useful and more rich functionalities, the book just overflew them compared to the previous Rails versions. It is really a regrettable downgrade in quality.