Take a look at Lavin… a drop in replacement for Rabbit MQ supporting AMQP 0.9.1, we are currently evaluating lavin and are very satisfied with the results. Their site has excellent resources to get started with Message Queues in Crystal.
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LavinMQ is itself written in Crystal and a product of 84codes, a top sponsor of the project. 
Here’s a talk from Crystal Conf last year with some insights into the engineering behind it:
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