If you order the book today, it _______ arrive tomorrow. There are a number of ways I would express this idea but I would not use b).I would say "may'. Here are some other alternatives: 1. If you ordered the book today, it should arrive tomorrow.- it's almost certain to arrive 2. If you order the book today, it may arrive tomorrow. - it is possible but not certain 3. If you ordered the book today, it might arrive tomorrow. - it is possible but not certain 4. If you order the book today, it could arrive tomorrow.- it is possible but not certain 5. If you ordered the book today, it would arrive tomorrow. - it is certain to arrive

Seems highly dependent on the context. Both words technically work in that sentence, but the delivery methods are highly suspect. "Would" makes sense with respect to it being a part participle of "will." The intent of the statement is meeting a condition will result in receiving a book by tomorrow, therefore "would" is used to imply conditional choice or possibility.