Trade Selector is a client-side Fabric mod for Minecraft 1.21.x that automates librarian enchanted-book rerolling.

What it does;

Bind a librarian and its lectern
Choose a target enchantment, level rule, and max price
Start automation and let the mod reroll trades until it finds a match
Stop automatically when a matching trade is found

Features;

Automated librarian reroll loop
Enchantment targeting
Level matching modes: Any, Exact, Range
Max emerald price filter
Configurable success sound
Optional lectern recovery movement
Safety checks for manual movement/focus changes
Works even when Minecraft is not the focused window

Important behavior;

Lectern breaking does not auto-select tools.
The mod breaks lecterns using your currently selected hotbar item. If you want faster breaking, keep your preferred tool selected before starting.

Keybinds;

K — Open Trade Selector GUI
V — Bind librarian
B — Bind lectern
N — Toggle automation

Disclaimer!
Use at your own risk. You are responsible for following server rules. I am not responsible for bans, suspensions, or other account penalties resulting from use of this mod.