WriteBookCommand. Usage: Get a Book and Quill and send the command to the chat: /writebook

WriteBookCommand.java

package io.github.ensamisten.forgient.command;

import com.mojang.brigadier.CommandDispatcher;
import com.mojang.brigadier.arguments.StringArgumentType;
import com.mojang.brigadier.context.CommandContext;
import net.minecraft.commands.CommandSourceStack;
import net.minecraft.commands.Commands;
import net.minecraft.nbt.CompoundTag;
import net.minecraft.nbt.ListTag;
import net.minecraft.nbt.StringTag;
import net.minecraft.network.chat.Component;
import net.minecraft.server.level.ServerPlayer;
import net.minecraft.world.item.ItemStack;
import net.minecraft.world.item.Items;


public class WriteBookCommand {

        public static void register(CommandDispatcher<CommandSourceStack> dispatcher) {
            dispatcher.register(
                    Commands.literal("writebook")
                    .then(Commands.argument("text", StringArgumentType.greedyString())
                            .executes(WriteBookCommand::writeBook)));
    }

    private static int writeBook(CommandContext<CommandSourceStack> context) {
        String text = StringArgumentType.getString(context, "text");

        ServerPlayer player = context.getSource().getPlayer();
        ItemStack heldItem = player.getMainHandItem();

        if (heldItem.getItem() == Items.WRITABLE_BOOK) {
            ListTag pages = new ListTag();
            pages.add(StringTag.valueOf(text));

            CompoundTag bookTag = heldItem.getOrCreateTag();
            bookTag.put("pages", pages);

            context.getSource().sendSuccess(() -> Component.literal("Text written to the book!"), true);
            return 1;
        } else {
            context.getSource().sendFailure(Component.literal("You need to hold a writable book in your hand!"));
            return 0;
        }
    }
}