The bug
When you edit the book and attempt to select part of text that is obfuscated, the game crashes.
Crash report
18w43b: [^crash-2018-10-24_22.27.19-client.txt]
Description: Rendering screen
java.lang.StringIndexOutOfBoundsException: String index out of range: 35
at java.lang.String.substring(String.java:1951)
at cor.m(SourceFile:670)
at cor.a(SourceFile:614)
at cxp.a(SourceFile:795)
at cjj.c(SourceFile:817)
at cjj.a(SourceFile:380)
at net.minecraft.client.main.Main.main(SourceFile:144)
How to reproduce
Open book and quill
Paste § to the book and add the letter 'k'
Continue writing, the text should consist just from randomly changing characters
Try to select obfuscated text
Linked issues
is duplicated by
relates to
Attachments
Comments

Can confirm when pasting §, then typing 'k', then adding text and selecting it.
You are not supposed to be able to add the § symbol and styling yet at all, so that's the bug. It will be prevented in the upcoming snapshot until we implement proper component-based styling.
When I try to paste the
§
character in the book it appears to be ignored. Is there something special to consider?