I agree with the parent. In your case, while you are taking on a bigger project, you are not being called upon to manage multiple resources, multiple dependencies etc. That's a large part of what project management is. It sounds like you're more after something that can act as a guide to software development. A nice, short, practical book is "UML Distilled" by Martin Fowler. While it is geared towards teaching the application of UML, it does so by describing it's various diagrams in context and showing how to use them yourself.
The book starts out with a description of the software development process advocated by Fowler, which is an iterative approach. My favorite nugget is in a section headed "When to Use Iterative Development" which begins, "You should use iterative development only on projects that you want to succeed." :)
There's a breakdown of the project phases and the book deals with requirements, risk management, planning and lots of other issues, and at the end there's a nice simple example.
If you already feel like your project is in a bit of trouble, time may be of the essence. At just 166 pages, "UML Distilled" is a quick read and takes a very pragmatic approach that should get you going in the right direction quickly.