In published books, fiction or non-fiction, each author has their own individual writing style and there are no rules!
I have absolutely no problem with the use of contractions in books of any kind. The only problem I have with the quoted sentence is the use of "I've" - not because it's a contraction but because it should read:
"It's now been two weeks since I returned from..." - "I", not "I've".
I have returned doesn't look archaic at all, it's just not necessary.
Student or Learner