More fun for the people in the library!
All knowledge is good knowledge, and you find it where you can. As you go around you learn from the things you do, the things you see, and the things you kill. You write everything down and those note teach you things that will keep you alive! What ever doesn’t kill you and whatever you see can only make you stronger!
When you choose this archetype at 3rd level, you start to write down all the things you see and do. When you make a sneak attack against a creature type, write that down. Every time after you make a non-sneak attack against that target, you gain a bonus to damage against that target equal to the number of times you have previous made a successful sneak attack against that creature type, up to your proficiency modifier. For humanoids, you must make sneak attacks against each different humanoid subtype.
Learn from your Friends
Starting at 3rd level, you start to write down what your friends do to learn from them. Every time an ally succeeds at a Intelligence (Arcana), Wisdom (Religion), or Intelligence (Nature) check, note that they succeeded. When you attempt a Intelligence (Arcana), Wisdom (Religion), or Intelligence (Nature) check, gain a bonus to that check equal to the number of times you have previously observed a successful check, up to half your proficiency modifier.
Starting at 9th level, you start to apply what you see quickly. Once per day, when you are the subject of an effect or spell from a creature that requires a save and you succeed, you can choose to learn that effect or spell. Before you take a long rest, you can use that spell or effect once. You regain this ability after a long rest.
Success Breed Knowledge
Starting at 13th level, you begin to learn from all the attempts on your life. Every time you succeeds at a save, note your success for that agility score. When you attempt a save in the future for the specific ability score, gain a bonus to that save equal to the number of times you have previously succeeded on that save, up to half your proficiency modifier
When you reach 17th level, you can use your useful observation ability up to three times each time choosing different spells or effects.