Free: https://www.cybrary.it/
Cheap: https://www.pluralsight.com/
The entry level cert in this area is the CEH. It's kind of looked down upon, like a lot of entry level certs are, but studying/working towards that isn't a bad thing.
Books:
- Practical:
The Web Application Hacker's Handbook 2nd Edition - Gives a very good overview and is a good place to start.
The Hacker Playbook 3: Practical Guide To Penetration Testing - #3 just came out. Haven't gone through my copy yet, but I've heard good things.
RTFM - Red Team Field Manual - Nice to have, quick reference guide
BTFM - Blue Team Field Manual - Like the above, but for the good guys ;)
- Covering the bigger picture, if you're curious (geopolitical):
The Hacked World Order: How Nations Fight, Trade, Maneuver, and Manipulate in the Digital Age
The Red Web: The Struggle Between Russia's Digital Dictators and the New Online Revolutionaries
Dark Territory: The Secret History of Cyber War
Cyberspies: The Secret History of Surveillance, Hacking, and Digital Espionage