You can get a library card for free if you are a citizen of Billings, Stark or Slope county. Otherwise there is a charge yearly for a card ($20/ year for a single or $25/year for a family).
What Skywatcher stated is accurate but to add to it, when you come in to get the card you will need a state issued ID and something that shows your current address such as a utility bill with your name on it.
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