Why would extra spell be relevant to a chameleon? It explicitly says they can prepare prepare arcane and divine spells from any list. I guess you could do a 2 level dip as a caster, but that doesn't seem like it would be worthwhile.
The exact wording is a little vague, but the whole "you prepare and cast these spells just as a wizard" plus the lack of something directly stating that you know every spell in the arcane book has lead my group to believe that you still need to have a spellbook with the spells written you want written into it. In other words, you don't get to cherry pick each day. Rather, you can learn any spell from any arcane list, but you still need to write them down and memorize them (hence why it specifically references using spellbooks).
Other groups may decide otherwise, but that is how it has been interpreted whenever it comes up in my group.
I speak AD&D to my fluff, 3.5 to my crunch and 5E to my players.