Q In D&C 39:6 It talks about the Holy Ghost revealing all the peaceable things of the kingdom. I thought the Holy Ghost revealed all things concerning the kingdom. What are the 'peaceable' things?

Level 2 - If we will receive Christ, He will give us His grace (see power) to become his sons and daughters. Thus we also become the sons of God (Christ's Father) by coming into the family of Christ. Just to make sure that we know what it means to receive Christ, we are told that to receive Christ, we must receive His Gospel (v5). And receiving His gospel means: correct faith which leads on to true repentance, baptism by immersion in water by a legal administrator, and baptism by immersion in fire. This last can occur once a person has received the gift of the Holy Ghost. This is the same pattern followed in Acts chapter 8 and 19.

First, some background. The Holy Ghost is a person of spirit. That is, he does not have a physical body as we do. He is the third member of the Godhead. He works directly with God the Father, and Christ to bring about the eternal life of man (Moses 1:39). It is the task of the Holy Ghost to testify of the truth, and bear witness to us of the Father and the Son. The Holy Ghost is capable of constantly communicating with us if we can learn to hear his still, small voice (1 Kings 19:11-12). It is a voice that is almost felt more than it is heard. Nephi, an ancient prophet on the American continent, tells us that Holy Ghost is given as a gift to those who diligently seek God (1 Nephi 10:17). Certainly the Holy Ghost has much to teach us about the way our Father in Heaven would like us to live our lives. But this is not all that he can testify about. John 14:26 tells us (among other things) that the Holy Ghost can teach us ALL things. He could teach us advanced physics or principles of geothermics if there were such a need in God's plan. Those topics might seem interesting (or dull) to us, but what the Holy Ghost really wants to teach us is about the love of God, and how we can qualify to enjoy his love forever. This is a fruit that is "most precious and sweet above all that which is sweet (Alma 32:42).

In answer to your specific question, I am not sure I know why the Lord chose to say that the Holy Ghost would teach us the "peaceable things of the kingdom" instead of saying "all things of the kingdom". But I have some ideas. One is that God our Father is Omniscient. He knows everything. He has all knowledge. There is no knowledge, but what he has it. He does not make educated guesses based on past experience, and he does not forecast. He does not decree what will happen, He simply knows what will happen. He knows everything that there is to know. That is a far cry from us. The fact is that we are nowhere near capable of receiving all that the Holy Ghost could teach us (John 16:12, D&C 50:40). But we can learn a little at a time (Isaiah 28:9-10). Our capacity to learn and receiving instruction by the Spirit is in direct proportion to the heed and diligence which we give to every bit of instruction that God gives us (Deut. 8:3).

My other thought is this. Some people think that they can draw near to God by endless studying (Eccl. 12:12-13). While study is good, intellectual tromping about in the scriptures yields little of substance unless it leads us to change our actions. We all know people who always have something interesting to say about the Greek translation of a verse, or whether the pearly gates swing or roll. They are always learning new facts, but never coming to a knowledge of Christ (2 Timothy 3:7, John 14:6). I think knowledge of Christ is the MOST "peaceable thing of the kingdom". Could the verse in D&C 39 refer partly to that? In contrast to the intellectuals, the soldier in Christ (2 Tim. 2:3) who obeys much, and has clean hands and a pure heart (John 1:47, Psalms 24:4) will be enabled to learn things through the Holy Ghost that the intellectuals may never learn. So to me, this verse says that regardless of the strength of our minds and our ability to study, the Holy Ghost will teach us what we need in the moment that we need it (D&C 84:85). This is our promise if we follow the steps above, and receive Christ into our lives more fully.

Personally, I love to study the scriptures. I never seem to get enough of it. I get caught up in enjoying the symbolism and spirit of them. This is a great blessing that has yielded me much. But if I could trade whatever knowledge I have gained for a heart that would never desire to sin; I would do it instantly. To me, this is one message in D&C 39; stop dwelling on what you think you know or don't know, and learn to hunger and thirst after righteousness. It might take a lifetime to learn that lesson completely, but the blessings it brings are wonderful.
I end with the words of Christ: "Blessed are they which do hunger and thirst after righteousness: for they shall be filled with the Holy Ghost." (Matt 5:6)