The invention relates generally crystalline mesylate salts, crystalline chloride salts and crystalline sulfate salts of the compound (S)-2-(3′-(hydroxymethyl)-1-methyl-5-((5-(2-methyl-4-(oxetan-3-yl)piperazin-1-yl)pyridin-2-yl)amino)-6-oxo-1,6-dihydro- [3,4′-bipyridin]-2′-yl)-7,7-dimethyl-2,3,4,6,7,8-hexahydro-1H-cyclopenta[4,5]pyrrolo[1,2-a]pyrazin-1-one that is an inhibitor of Bruton’s tyrosine kinase. In some aspects, the crystalline salts are single polymorphs.