I keep trying to read and understand the attacks that have all happened so far... after you understand the vectors that are currently in use, hopefully it'll stoke your imagination to see how you might attack other systems and make up new vectors. All the current attack write-ups will usually link to vulnerabilities as well. See HackerOne disclosures, Google Project Zero or the "How I hacked X" disclosures you see on HN.
I don't think published textbooks are very useful — attackers also have access to them, and if the attack has been written down it'll likely be encoded into a firewall software or security process rulebook already (though it might still work for smaller companies lagging behind on the curve).
I don't think published textbooks are very useful — attackers also have access to them, and if the attack has been written down it'll likely be encoded into a firewall software or security process rulebook already (though it might still work for smaller companies lagging behind on the curve).