Answer: Let me start my reply by saying that the challenge of coaching little kids football is not in the Xs and Os. The challenge for the coach is in making the game and the practices fun for all the players. It is very easy for a coach to get caught up in "teaching" the kids to play and in winning that fun is sacrificed. You need to adjust your expectations and never forget that the players are little kids playing a game for fun.
It's important to keep every thing as simple as possible and don't assume your players know even the basic things. Most seven year olds do not know the difference between offense and defense, as I am sure you will find out.
Let's face it... Whatever your knowledge of the game, I'm sure you know more about football than any 7-year-old. Therefore the mantra Keep It Simple is essential.
Remember that kids are not familiar with football terminology and jargon. Don't get too technical with them about, assignments, plays and formations. In our development program, I limit defenses to a basic 6-3 defense (i.e. 6 defensive lineman, 3 linebackers and 2 defensive backs). This makes learning offensive blocking assignments less of a problem for the kids because there is always a man over each of the offensive linemen.
Most books on coaching football do not take into account the age of the young players and what may be appropriate for a group of 10-year olds is probably not appropriate for 7-year olds. Common sense is the coach's best tool.
Without seeming self serving, I would recommend that you read my book "Mom, Can I Play Football?" It will explain some of the pitfalls you will need to avoid and give you a starting point for dealing with the younger kids.
You didn't say anything about the program you will be involved with. The program and its philosophy are extremely important. Youth football programs should have special rules that ensure player's safety and rules that require participation by all. Unfortunately not all programs have such rules. Even with the right philosophy and the right rules, the approach of the coach is critical to a successful experience for all the players.
To, directly answer your question about a recommended book, there are lots of "coaching football" books ("Coaching Youth Football" by the American Sport Education Program is just one) that provide diagrams of plays and lots of different defenses. My experience is that any book must be viewed as a guide for possible experimentation based on the specific ages and skill levels of the players. The basic techniques are what is important and while many books discuss some of the basics, most go quickly beyond that to more advanced levels.
You might want to visit coachpaterno.com They often have some interesting information there that might be of help but remember that little kids need special consideration.
I hope this helps.