Reuters reports that Bain Capital, Blackstone Group and Thomas H. Lee Partners, the former private equity owners of Houghton Mifflin Co., are bidding for Thomson Corp.‘s text book publishing unit, sources close to the process said Monday, in a deal that could be worth around $5-billion. A separate source told Reuters that Bertelsmann is considering teaming up with that bidding group, and yet another source indicated Apax Partners and Warburg Pincus are also involved in the auction with the two buyout firms bidding separately for the unit. All five private equity firms have placed preliminary bids and are meeting with Thomson executives, and it looks likely that some combination of firms will win out – just a matter of which grouping seems the most attractive…