Q7. What are the decrees of God?
A7. The decrees of God are his eternal purpose, according to the counsel of his will, whereby, for his own glory, he hath foreordained whatsoever comes to pass.
Q8. How doth God execute his decrees?
A8. God executeth his decrees in the works of creation and providence.
Q9. What is the work of creation?
A9. The work of creation is, God's making all things of nothing, by the word of his power, in the space of six days, and all very good.
Q11. What are God's works of providence?
A11. God's works of providence are his most holy, wise, and powerful preserving and governing of all his creatures and all their actions.
Again, I'll repeat the recommendation of John Frame's The Doctrine of God. Professor Frame more than adequately covers all of the topics that we talked about under the doctrine of God including the decree.
Regarding the doctrines of grace, since unconditional election is the first of the letters from the TULIP acronym that we've addressed, I'm just going to recommend two books that cover this topic and everytime we discuss one of the doctrines of grace I'll note the relevant chapter. This is mainly because there are a large number of introductions to presbyterian/reformed/calvinist theology that cover the doctrines of grace. Many of them are pretty good. These are the best two that I've read in terms of being complete, orthodox, and applicable and so rather than advise people to buy an even larger number of books I think that these two (particularly the Horton book) will be more than sufficient though I'd be happy to recommend more if people ask.
Putting Amazing Back Into Grace: Embracing the Heart of the Gospel by Michael Horton - Dr. Horton is probably best known as the host of the Christian Radio program, the Whitehorse Inn. He is professor of theology and apologetics at Westminster Seminary California and the author of a number of books addressed to various audiences. This book is concentrated on introducing readers to the basics of the Reformed faith (and I recommend it above the following book simply because it covers more than just the doctrines of grace). Dr. Horton discusses the subject of unconditional election in chapter four, "Grace before time."
What's So Great About the Doctrines of Grace? by Richard Philips - Reverend Philips is the senior pastor of Second Presbyterian Church in Greenville, SC. He has a pastor's heart and a seminarian's mind. I like this book about the doctrines of grace because Pastor Philips is focused on developing the doctrines through faithful exposition of Scripture, on explaining why these doctrines should lead the believer to worship our sovereign God, and finally on showing how these doctrines are important for the Christian life. Pastor Philips deals with unconditional election in chapter three.
Second, here is a quick survey of where the decrees of God are discussed in Reformed systematic theologies. John Calvin principally deals with the decrees of God in Book 1, Chapters 16-18 of his Institutes of the Christian Religion and then election in Chapters 4-6 of Book 2 and 21-24 of Book 3. Herman Bavink addresses the decrees and election in Chapter 7 of the second volume of his Reformed Dogmatics (God and Creation) and then God's providence in Chapter 4 of the same volume. Charles Hodge discusses the decrees and providence of God in chapters 9 and 11 respectively of his Systematic Theology (available free here as a .pdf file). A.A. Hodge deals with these doctrines in Chapters 9, 10, and 13 of his Outlines of Theology (available free at Google books and for sale at the Shady Grove bookstore).
Finally, here are some articles that you can read for free online:
"On the Predestination of the Saints" by Augustine - Helpful to see that the doctrine did not originate in the 16th century.
"Predestination and Free Agency" by W.E. Best
The Reformed Doctrine of Predestination by Lorraine Boettner
"Of Election to Everlasting Life" by Thomas Boston - A sermon by an old Scottish Puritan Presbyterian.
"Electing Love" by Robert Murray M'Cheyne - A sermon on John 15:16 by a gifted Scottish Presbyterian minister (see a post on Pastor M'Cheyne further down in the blog or Andrew Bonar's biography of his life available at the Shady Grove Bookstore).
"Calvin, Dordt, and Westminster on Predestination: A Comparative Study" by John Murray
"The Sovereignty of God" by John Murray
"The Doctrine of Election" by A.W. Pink
"Election" by Benjamin B. Warfield