Welcome!
Your account was created nearly 3 years ago, so not sure about the "first time" thing, and most brand new players wouldn't get 3 characters to 110+ in a week, I guess maybe you've taken on a friend's account, or had someone higher level group you and boost your experience. As a brand new player I'd normally reccomend starting from scratch to get the basics, but it sounds like you've sort of covered that already. There is an NPC that gives a list of suggested hunting locations for low levels, but you have already surpassed all the suggestions which is normally quite an interesting part of the game, which is a shame.
A key part of this game is discovery and exploration, so it's intentionally light on guides, with of course some exceptions for things like arcamon
https://mirarcadia.com/guide/arcamon/
If you press A you will see an achievement list, which in turn has a list of bosses and monsters. They are vaguely (but not exactly) in order of difficulty, with the easiest first. You should try working down the list until you get to stuff you are unable to kill yet, and consider killing those to be a goal to improve towards.
Bosses (and monsters, with much lower rate) will tend to drop items and books that are equal in level to them or a little lower. For example Iku Turso is level 116 and will typically drop things between level 100-120.
If you hover your cursor over a target, it will show it's level near the bottom right of your screen.
Remember to take your time, enjoy exploring the game and don't rush to "catch up" with players who have been playing for years, all you'll end up doing is skipping fun and interesting content and will get bored quicker.
For low level books, it can often be frustrating with the amount of time it takes to find them, the drop rates have been improved but it will still take a lot of kills, and it may be more time efficient to simply kill lots of monsters while levelling and collect things to sell as you go, sell them to shop and you will make a lot of gold, which you can use to buy the books and items you need.
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I should add, that killing a boss that is around the same level as you is generally extremely difficult or even impossible depending on your class. You'll need to either get a group together of similar level players and use tactics to kill it, or restrict yourself to killing things that are lower level than you, which really limits what useful items you are likely to get.