I have read two of Robert G. Allen's books, and they both provided good, general information. Although they did not get very specific about any single topic, I found them useful since they helped me generate ideas about making money.
The first book I read, Multiple Streams of Income, is a guide to making money through a variety of means, such as through stock investments, real estate, tax liens, networking marketing, tax savings, and selling information products, to name a few. It is a great book if you want an overview of the many ways you can make money (aside from having a job). In fact, if you are considering a part-time income to supplement your job, and are unsure of how you want to make more money, this book will give you a ton of ideas.
The second book I read was Multiple Streams of Internet Income. I found this book less useful, mostly because by the time I read it, I had already run my own Internet business for a few years and didn't find much that I didn't already know. But it did contain solid information and is a good introduction to the many ways to make money online.
You may also be wondering about Robert G. Allen's real estate seminar, "Creating Wealth with Real Estate." Robert Allen tours the country and gives 2 hour presentations that encourage you to attend his full 3-day real estate seminar. The cost of the full seminar is $3995. I have not attended this seminar, but if you want to get an idea of Robert Allen's real estate investing techniques, there are two chapters on this subject in the Multiple Streams of Income book, one of them dealing with flipping while the other deals with foreclosures. I believe these are some of the same techniques that are taught in depth at his seminars. He has also written a book called Nothing Down in the 2000s, which you can buy at any bookstore. I strongly recommend you read his books and make sure you are interested in getting into this business before investing almost $4000 on his seminar.