Start with Without Remorse, basically about John Clark. Clark is the leader of R6. The book titled Rainbow Six is also a great read and is the 2nd in the John Clark series.
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Without Remorse (John Clark...
A Game of Thrones / A Clash of Kings / A Storm of Swords / A Feast for Crows / A Dance with Dragons (Song of Ice and Fire Series) (A Song of Ice and Fire) Set of 5 books, Pack of 5