I was in a very similar situation this summer with my 4-year-old. He, too, was acting up and just generally being incredibly hard work; again, this was for very good reasons (having a new brother, finishing daycare, anticipating moving house--albeit locally, and having his dad work 6 hours away from home for most of the week). But gosh, it was hard! I had been so looking forward to the summer with my little boy and our new baby, as I was able to take 4 months off work, and then here I was, exhausted from lack of sleep, no family nearby, my husband away, and my 4-year-old being impossible. All of which is to say that I sympathize.
I second the recommendation for How to Talk so Kids Will Listen. It contains some very useful strategies. But myself, I would start with Alfie Kohn's Unconditional Parenting. There were a few times when I didn't entirely agree with him, but I realized after I'd finished that it had effected a fundamental shift in the way I saw my role as a parent and my relationship to DS. I think I became an awful lot more sympathetic to his meltdowns, and began to see them not as him 'misbehaving' but as him both processing his emotions and communicating to me that something wasn't going right and he just didn't know how to handle it. That "something" might have actually been incredibly minor to me, but significant to him--Kohn's book made it much easier for me to see things from DS' perspective. I'd always thought I was a loving and sympathetic parent, despite the things I knew I could do better; but it wasn't until I read Kohn's book that I realized I hadn't always been able to put myself in DS' shoes, and how much of an effect this had on my ability to parent him calmly and with respect.
To sum up, I found the philosophy that's sketched out in this book really convincing, and it has empowered me as a parent. How to Talk So Kids Will Listen didn't effect such a paradigmatic shift in my thinking, but it did provide me with some useful strategies (even if I didn't agree with them all).
PS For what it's worth, I'd looked into parenting books before (eg Playful Parenting and others) but these didn't really help me when DS was having a meltdown. In your situation, I'd really recommend UP and How to Talk. You can get a sense of whether you'll appreciate their approaches by reading the introductions. Good luck!