The Humble Book Bundle: Java is presented by O'Reilly Media and is available for a little less than two weeks from publication of this post. Purchasers of the bundle can select which "level" of bundle they'd like to purchase. The smallest bundle requires a minimum payment of $1, the middle bundle requires a minimum payment of $8, and the largest bundle requires a minimum payment of $15 (all currencies expressed here in U.S. dollars). These bundles are of Java-oriented electronic books published by O'Reilly and include some impressive titles. Portions of the proceeds go to charity, the electronic books are DRM-free, and the electronic books are available in multiple formats (PDF, EPUB, MOBI).
Tier 1 Bundle ($1 Minimum)
The first tier bundle can be acquired for as little as one dollar and includes the five books Think Java, Java Generics and Collections, Java in a Nutshell, 6th Edition (covers Java 8), Java Pocket Guide, 4th Edition (covers Java 8 and 9), and Java Message Service, 2nd Edition (covers JMS 1.1).
Tier 2 Bundle ($8 Minimum)
For at least $8, the second tier adds the following books to the books available in the first tier: Learning Java, 4th Edition (covers Java 7), Java Threads, Third Edition, Java 8 Lambdas, RESTful Java with JAX-RS 2.0, Second Edition, and Minecraft Modding with Forge.
Tier 3 Bundle ($15 Minimum)
For a minimum payment of $15, all three tiers are "unlocked," providing access to the ten electronic books associated with the first two tiers and with these five electronic books added: Java Network Programming, 4th Edition, Java Web Services: Up And Running, 2nd Edition, Java Cookbook, Third Edition (covers Java 8), Java Performance: The Definitive Guide, and Jenkins: The Definitive Guide.
I already have the printed editions of some of these books and they are excellent. There is also a nice mix of books here with some designed for beginners (such as Think Java and Learning Java), some designed for more experienced Java developers (such as Java Generics and Collections and Java Performance: The Definitive Guide), some designed for general Java (such as Java Cookbook and Java in a Nutshell) and some designed with focus on recent Java (such as Java 8 Lambdas).
It could be argued that any one or two of these electronic books are worth $15 USD, but getting all 15 electronic books for that price is a great deal. When one considers that these electronic books are DRM-free and come in multiple formats and that a portion of the proceeds goes to charity, it's even better.