It's probably been mentioned before and obviously not high on the priority list but if Turbine happens to read these suggestions:
Could a wizard keep their spell book after they TR? I know it would probably goof things up if a wizzy TRed into a fighter and it was scripted to keep their book; this is my guess why it's not done.
It's less problematic for a sorcerer, in my opinion, as the way they choose spells doesn't get affected. A wizard must find/buy all those spells again, scribe them and if anything it's just slightly annoying. (Side note: Please let me see spells I've scribed of a higher level. If I'm character level one and i choose to scribe a caster 9th level spell I cannot see if I have scribe it or not which again is a minor annoyance.)
A possible solution to the spellbook issue would be to create an item in their inventory that's the actual spell book. This would keep in line with d&d. A double click to open would access their spells screen.
Yes, it would take up one inventory slot which may or may not be to most people's liking. We could always add a slot to the character's paperdoll like the quiver slot, add a spellbook slot. (Perhaps we could even get fancy and have special spellbooks from raids with enchantments on them: +10% casting alacrity, for example. Although I'm not sure how this would work for sorcs and divines who wouldn't "use" a spellbook but perhaps we could over look those semantics and script it in?)
All spells scribed would go into this item and it would be kept even if they TR into a fighter, though the fighter would not be able to use it without wizard levels. Set it to class Restriction: Wizard (Not sure if this part would even be necessary)
Thanks!
TL;DR version:
Give spellbooks. Keep all spells on TR. Make item slot with spellbook. Possibly enchant this item.