All small signal tubes are relatively cheap to the power ones and UNfortunately the power tubes have less life than small signal. I bet that replacing 12ax to the different kind of tube will not strike your budget too hard as you only need to change your tube I guess in 2...3 years.
I'm 100% sure that there are plenty of great books that step you thorugh complete understanding from basics of electronics to the complete components including amps, preamps, tuners and others, but I was studying electronics on different language and cannot recommend you a specific one to look arround.
All I used to do some time ago is to open the amp or preamp and study with open box to be well oriented with PC-board or elements v.s. circuit i.e. looked in the circuit like a map and than finding a proper "directions" through the real circuit elements.
Now I hope that you don't stack this preamp on ANY component and don't stack ANY component on your preamp as well.