There are two principles we can apply when deciding which books or authors are appropriate. First and foremost the authors need to have a high view of scripture. In establishing what is considered a high view of scripture there also a few things to consider: the nature of inspiration, and methods of interpretation. Seventh-day Adventists reject higher critical forms of interpretation and do not believe that we should utilize methods that impose the same critiques applied to other books besides the bible. When it comes to the nature of inspiration there are generally three schools of thought: verbal, plenary or thought, and moral inspiration. We believe in plenary or thought inspiration and therefore we could not reduce the bible to simply a collection of moral literature. While we do do not believe that the bible is literally the words of God I believe that we would have more in common with those that believe in verbal inspiration. Evangelicals generally believe in verbal inspiration and as a group I believe would come closest to agreeing with us in interpretation and theology.
The second criterion for establishing whether an author should be read concerns not just the method of interpretation or the understanding of biblical inspiration but in the content. I believe that in order for Seventh-day Adventists to find something valuable in other Christian authors those authors need to be adding to our understanding of the bible. Because Adventists have a very developed understanding of the Great Controversy not all evangelical writers will meet the standard of adding something to our knowledge. However I do think that Christian authors can add in our understanding of the history, events and culture of the biblical world and from time to time they can help us in spiritual application. But I would think that the primary way that Christian authors could add to our understanding would be in developing our understanding of the biblical world.
In conclusion I believe that authors that have a high view of scripture, including verbal and thought inspiration, and that can add to our understanding of the bible are a compliment to our study and Christian development. I have generally seen that this is limited to evangelical authors with the occasional exception. But as time goes on I think that there will really only be two sides: Babylon and the Remnant.