ClickAutomation.com is a site dedicated to automation products. One of the nice extras available is an overview of the various proximity sensing technologies. The article, written by Baumer Electric and offered as a PDF, explains the basic technologies used for inductive, capacitive, photoelectric, and ultrasonic proximity sensors. While it isn't an exhaustive comparison of the sensor types, it can at least help you decide which technology may fit your application best.
2. Engineering Podcast
If you're in the mood to listen to something engineering-related, check out IEEE Spectrum's podcast. Typically each monthly episode runs between half an hour and an hour and covers several topics. There are also additional episodes for special reports or on stand-alone topics. You can subscribe to the podcast using the podcasting software of your choice; alternatively you can listen online.
3. The Motherlode of Online Tips
Mark Fraunfelder is the author of the book "Rule the Web", an excellent compendium of tips, tricks, tools and information to help you bend the Internet to your will. Whether it's how to optimize your site to help search engines find it, score coupons for Internet purchases, manage downloads, and make your browsing experience more secure, you'll find that information in the book. The related Web site (which I'm linking to here) includes a number of 60 second audio files that highlight the various tips from the book and links to Rule the Web podcasts.
Vibration Monitoring, Testing, and Instrumentation
Editor: Clarence W. de Silva
Publisher: CRC Press
Page count: 694
This hardcover book, intended as an authoritative reference for engineers, technicians, designers, researchers, educators, and students covers both theoretical and practical information on the monitoring, testing, and instrumentation of mechanical vibration and shock.
There are fourteen sections, each of which is authored by one or more of the various contributors. The sections are, in order: Vibration Instrumentation, Signal Conditioning and Modification, Vibration Testing, Experimental Modal Analysis, Mechanical Shock, Machine Condition Monitoring and Fault Diagnostics, Vibration-Based Tool Condition Monitoring Systems, Fault Diagnosis of Helicopter Gearboxes, Vibration Suppression and Monitoring in Precision Motion Systems, Vibration and Shock Problems of Civil Engineering Structures, Seismic Base Isolation and Vibration Control, Seismic Random Vibration of Long-Span Structures, Seismic Qualification of Equipment, and Human Response to Vibration.
There are many illustrative diagrams and illustrations, along with sections of boxed text that highlight the important points discussed in each sub-section. Should you need more information, the references are listed at the end of each chapter. It's not a particularly sprightly read—it has too academic a tone for that—but it contains a wealth of good, relevant information presented clearly and discusses both the theoretical underpinnings of the subject as well as its more practical applications.