• Some (very elementary) notes on categories in PDF
  Unfortunately, these notes contain no discussion of Yoneda's Lemma or
  limits and colimits in categories. Thus they cannot even be called a proper
  introduction to category theory; probably, they may only be useful as a
  starting point for someone who never encountered categories before.
• Notes on the structure of finite fields in PDF
• Introduction to character theory for finite groups in PDF
• The main theorem of Galois theory in PDF
  

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Other potentially useful materials:

Detailed explanations of how one can think about monoidal categories,
and especially associativity constraints: notes in PDF