$FREE = Lessons In Electric Circuits - 6 PDF books, 2700 pages.
$FREE = Electricity & Electronics Training - many older modules, scroll down for links to other modules.
$FREE = Engineer's Mini Notebooks - many older booklets.
$20 = Make - Electronics , 2nd edition, 2015.
$30 = Make - More Electronics , 1st edition, 2014.
$21 = Practical Electronics for Inventors , 4th edition, 2016.
$23 = Complete Electronics Self-Teaching Guide with Projects , 4th edition, 2012.
$97 = The Art of Electronics , 3rd edition, 2015. The 2nd edition is great too.
I went through Electronics Engineering and built a Midi-CV converter for a eurorack module as a final project. Here's the resources that helped me during that time:
Make: Analog Synthesizer's By Ray Wilson
Music From Outerspace
Electronotes.->I highly recommend the MUSICAL ENGINEER’S HANDBOOK and ELECTRONOTES BUILDERS GUIDE AND PREFERRED CIRCUITS COLLECTION sold there.
Art of Electronics. ->This will be your bible during school.
Eurorack Mechanical Spec
Eurorack Electrical Spec
Try and build a DIY kit. That will definitely be an easy transition (based on difficulty of the DIY) into the process of building modules/synths.
Refer to the resources pinned to this subreddit.
I would also highly recommend getting to know your professors. If they're teaching electronics, chances are one of them might be into "synths" as well.
Print: The Art of Electronics
Ben Eater - Builds his own 8 bit computer and walks you through all the steps
http://www.allaboutcircuits.com/textbook/ This might be a good place to get started, I am taking a physics electronics advanced lab as an undergraduate in physics, and I have found this textbook to be pretty useful. Also Hororwitz's the art of electronics is probably the best text on electronics, however very dense (1200 pages). There is definately a lot to learn, and this is just getting you started in the electronics of it... idk specifically about tube amps though, but understand circuits is probably going to be a must
link to buy horowitz: https://toptalkedbooks.com/amzn/0521809266
with the lecture videos for EGGN 281
Art of electronics