PHASE

Orange has an excellent set of materials called PHASE. Their slogan inverts common parental frustrations, and claims, "It's just a phase...so don't miss it." The Phase materials help you gain insight into the needs youth have in various stages. This lets you adjust the way you relate to them. 
To put it another way, Phase is really helpful in reminding me of the difference between a 9th grader, and an 11th grader. 

You can see their website here. There's lots of free content! If you're working with almost no funding, I'd recommend getting the book. It's a bit awkwardly laid out, I think. However, start by wrapping your head around the charts in the book, and it will help you a lot. Much of what we talk about here at YOUTH MINISTRY: SMALL CHURCH comes from these materials.


Untangled

This book is fantastic. It's the most helpful, concrete book I've found on adolescent development written by a psychologist. It's about girls specifically, written by a Psychologist who also works as a high school counselor. Lisa Damour, PhD, the author, is convinced girls develop by addressing roughly seven big transitions. Each chapter discusses one of those transitions in a crazy helpful way, and gives tons and tons of concrete, practical advice about how to help guide girls through these transitions. Here's the book on Amazon. My only regret about this book is that I'm unaware of an equivalent for boys that is well done as Untangled.