Yes, I own bitcoins right now (no idea about my co-author.) We are not journalists, just technical book writers. Of course we are probably biased, in the same way that someone writing a stock market book invests in the stock market, or someone that writes about vegetarianism is probably a vegetarian.

When I write newspaper opinion pieces on bitcoin (only one so far) I make sure it says in there that I own bitcoins.

I definitely agree though that bitcoin raises new questions about conflict of interest when writing about technology.

Are people who instead hold USD, EUR, or CNY unbiased when writing about those currencies or BTC?

I would agree authors should disclose their participation, but I think disinterest/divestment on such matters is an unrealistic goal. Any stake or non-stake implies some personal preferences, and some personal stake is practically a prerequisite for motivating enough attention to understand and explain.

(Or as VC John Doerr has said: "No conflict, no interest".)

Co-author writing here. Although we think Bitcoin has (probably) a bright future, the book is really just about explaining how it works (the protocol and the cryptography behind it).