Twilight could weather the loss of one of her many books on magic, so long as it's not that particular one on forbidden transformation magic. But what's the chance of that happening with an efficient assistant like Spike around?
Manehatten, for all intents and purposes, is the safest place to live. There's walls. Protection. The Wasteland doesn't seem to have an influence on what happens within the district. All is safe and well, there is nothing for anypony to worry about.
After Tirek released Twilight's friends, instead of letting Tirek steal all her alicorn magic, Twilight teleports her friends to a safe place and resumes her intense batle with Tirek...