What's a good reference for learning about recombinant proteins?
i would start by searching PubMed literature database for relevant articles using this query:

"protein"[Title/Abstract] and "expression"[Title/Abstract] and "purification"[Title/Abstract] and "recombinant"[Title/Abstract] and "mammalian"[Title/Abstract] and ("E. coli"[Title/Abstract] or "CHO"[Title/Abstract] or "Sf9"[Title/Abstract])


which returns 116 articles (including 7 reviews).