Hi.
A good call IMHO. There will always be a shortage of techs, especially those who are also musicians.
I don't know any good books about SS technology, but there's quite a few on tubes. That link is in my other computer, I'll look it up.
The reason for pushing You towards tubes is simple. There's virtually no schematics for SS amps of today, and the designs of SS amps is generally not tech friendly if there's no schematics.
Tube amps on the other hand are generally rather straight forward, so it's likely that someone traces the schematics even if it's officially unavailable.
Even though You speak of a hobby, you'll soon find yourself in a demand if you can resurrect dead amps

.
As an electrician You also have one advantage over most hobbyists: You have the licence to repair electrical appliances, so it's legal.
Regards
Sam