As for if it is cosher to vapourise the wizards spellbook...sure, if they were dumb enough to put it at risk, they deserve what they get.
How much work does a wizard have to put into protecting their spellbook in your games (not just aglondier's, I mean everyone's)?
As for a backup of my spellbook...100gp to copy my single 1st level spell...or 12.5gp and 1xp to Scribe Scroll...
At first level, with 19 cantrips and 7 1st level spells...so for 206gp 2sp 5cp and 26xp you have your entire collection of spells saved for a future emergency.
sure, at higher levels it isn't as cost effective, but by then I have better means at my disposal anyway.