Here's the error
net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Bookshelf (bookshelf) Caused by: java.lang.IllegalAccessError: tried to access field net.minecraft.client.resources.I18n.i18nLocale from class net.darkhax.bookshelf.client.RomanNumerals at net.darkhax.bookshelf.client.RomanNumerals.mapLevels(RomanNumerals.java:50) at net.darkhax.bookshelf.client.RomanNumerals.insertRomanNumerals(RomanNumerals.java:43) at net.darkhax.bookshelf.Bookshelf.onLoadComplete(Bookshelf.java:133) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at net.minecraftforge.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:639) at sun.reflect.GeneratedMethodAccessor11.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) at com.google.common.eventbus.EventBus.post(EventBus.java:217) at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:219) at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:197) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:91) at com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:150) at com.google.common.eventbus.Subscriber$1.run(Subscriber.java:76) at com.google.common.util.concurrent.MoreExecutors$DirectExecutor.execute(MoreExecutors.java:399) at com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:71) at com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:116) at com.google.common.eventbus.EventBus.post(EventBus.java:217) at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:136) at net.minecraftforge.fml.common.Loader.initializeMods(Loader.java:757) at net.minecraftforge.fml.client.FMLClientHandler.finishMinecraftLoading(FMLClientHandler.java:336) at net.minecraft.client.Minecraft.init(Minecraft.java:535) at net.minecraft.client.Minecraft.run(Minecraft.java:378) at net.minecraft.client.main.Main.main(Main.java:118) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) at net.minecraft.launchwrapper.Launch.main(Launch.java:28) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at net.minecraftforge.legacydev.Main.start(Main.java:86) atnet.minecraftforge.legacydev.MainClient.main(MainClient.java:29)`

This looks like the access transformers from Bookshelf are not being applied. How are you defining the dependency in Gradle?

You can refer to Dark Utilities if you need a working example.

Hi there, thank you for your quick reply!

I am simply defining it as so:

repositories {

    maven { url 'https://maven.mcmoddev.com' }
}

dependencies {
compile "net.darkhax.bookshelf:Bookshelf-1.12.2:2.3.590"
    compile "net.darkhax.gamestages:GameStages-1.12.2:2.0.123"
}

I tried replacing compile with deobfCompile but got the error 'could not find the method deobfCompile()
DoI need to add anything else?

Update on the situation: I've tried adding it as an external Jar through my IDE and I'm still getting the same error.

For now I'm using a somewhat hack method of just manually adding the AT myself, as I feel this issue isn't with your mod, I apologise for bothering you and look forward to using Game Stages in my mod! Thank you for creating such a neat system.

You may need to use deobfProvided. Item Stages uses both GameStages and Bookshelf in dev fine. Forge ATs definitely have some wonky behavior though.

For some reason that worked, thank you for your help I really appreciate it.
Have a nice day!