We registered this great domain in year-2003 making name well-aged and giving extra value. There are more than 20,000,000 results in Google on a "open book" search. Domain gets regular internet traffic from people looking for openbook and word is popular. It's a great name for a business or start-up in misc categories.
Some popular related searches (perhaps due to its prior usage as a financial website) are on; trading, investing, trading commodities online, forex trading, open store, open advice, best book, best stocks to invest in, best stocks to day trade, penny stocks to trade, the market, trading futures, investment tips and more.
Openbook.org is available and on-sale due to its owner retiring soon. In fact, a desire for retirement income is a reason the owner can do an interest-free monthly payment plan (and a buyer's benefit).
P.S. No worries about using the domain while paying. You will have the same use as if you paid purchase price up-front, and be able to use your own DNS immediately upon making your first payment.