JOEL - The Apostles and other General Authorities(GAs) writing and selling books doesn't bother me at all. What really bothers me is the stuff that some of the rank-and-file members of the church are trying to make a quick buck on.

There are a few reasons why I believe the GAs write and sell books. First of all I am certain they sincerely believe that, as apostles and prophets, who have been given a responsibility to preach the Gospel to the world, they have something important to say; not just to the church, but to the whole world of people who might buy their books and read them. They are a great missionary tool.
President Hinckley's "Standing for Something" was on the top ten best-seller list, sold millions of copies worldwide and reached a great many non-LDS people.

Secondly, although they do receive a living allowance for their service in the Church, they might have no other significant source of income to pay for things not related to church work. As parents and grandparents I am sure they would like to be able to provide help for their offspring when needed. The need for this extra income would vary greatly depending on the individual financial situation. I know that some of them do not keep the profits they make in selling their books. They are either donated to the Church or other charities. For example the proceeds from "A Marvelous Work and a Wonder" written by LeGrand Richards all went towards the missionary work of the Church.
A story on BYU News Net describes how general authorities decide to write a book and what they do with royalties. Here is the link:
http://newsnet.byu.edu/story.cfm/39510
Their books are usually very reasonably priced, probably barely enough to cover the publishing and printing costs. A General Authority may write only one or two books in his lifetime, if any, so he is not really going to get rich off the 10%-15% royalties he might receive for a book which may only be purchased by a small niche of church members.

Lastly, I truly believe that as prophets of God, I can trust that their motives for writing the books are pure and with an eye single to the glory of God and for the benefit of His children on this earth.