Using this inside an event handler

Indeed, the Definitive Guide is wrong in that case.

I found a reference in the HTML5 event handler processing algorithm:

Invoke callback with one argument, the value of which is the Event object E, with the callback this value set to E's currentTarget.

The DOM level 3 event specification didn't say much about it in previous versions - it was meant to be language agnostic. The Appendix F: ECMAScript Language Binding just stated

EventListener function: This function has no return value. The parameter shall be an object that implements the Event interface.

However, current versions omitted these bindings. And in its Glossary appendix event listeners are described:

event handler, event listener: An object that implements the EventListener interface and provides an EventListener.handleEvent() callback method. Event handlers are language-specific. Event handlers are invoked in the context of a particular object (the current event target) and are provided with the event object itself.

Also, the upcoming DOM Level 4 draft, whose goals contain aligning the DOM with the needs of EcmaScript, does explicitly state in the Dispatching Events section:

If listener's callback is a Function object, its callback this value is the event's currentTarget attribute value.

In an event handler for an element, with default capturing (false), this will refer to the element which detected the event. Either one may be used.

For example:

element.addEventListener('keydown', function (event) {
    // `this` will point to `element`
}, false);

When capturing an event (true), say at the window level, event.target, will refer to the element which originated the event, while this will refer to the capturing element. For example:

window.addEventListener("error", function (event) {
    event.target.src = 'some_path';
    // `this` will point to window
    // `event.target` will point to the element that had an error
}, true);

I hope this exemplifies the difference between each.