Crash occurs on opening the Lexica Botania with Botania 1.5-161
http://pastebin.com/zgPkebgw

Change the openGuiClass for the alfheim book to vazkii.botania.client.gui.lexicon.GuiLexiconIndex in the json. I think. If that didn't work on it's own, trying using a dev release: Where I removed my hijacking the lexicon, since vazkii fixed it: http://joshiejack.uk:8080/job/Enchiridion%202/

(I May as well release those fixes as a c version... since i'm working on other things for a while :P )