Humble Bundle offers downloads of its Humble 4-Book Cybersecurity eBook Bundle for whatever you want to pay. If you opt to pay the bare minimum of $1, you'll get "Social Engineering: The Art of Human Hacking", "The Web Application Hacker's Handbook: Finding and Exploiting Security Flaws", "Practical Reverse Engineering: x86, x64, ARM, Windows Kernel, Reversing Tools, and Obfuscation", and "Threat Modeling: Designing for Security". That's $108 under the lowest price we could find for these eBooks purchased separately elsewhere.

Note: If you pay more than the average price (currently $8), you'll also get five more books (including "The Art of Deception: Controlling the Human Element of Security"). Finally, pay $15 or more to unlock another five books (including "CEH v9: Certified Ethical Hacker Version 9 Study Guide"). Deal ends July 31.