Comment by i7l

The easiest book that's not popsci but actual physics is Barton Zwiebach's "A First Course in String Theory".

It does not presuppose a background in QFT. It does require you to know quantum mechanics. Mind you, it's not as deep as the standard texts (Polchinski et al. or Kaku's work prior to going off the rails) or my favourite which is the two-volume "Quantum Fields and Strings: A Course for Mathematicians". But it makes reading the others possible.