I'm surprised there isn't a thread about it here already.
Currently, O'Reilly offers eBooks about JavaScript, all for half the regular price:
http://oreilly.com/store/dd-HALFD.csp
They also have a (random?) eBook reduced by 50% each day.
(Actually, two - one from themselves, one from Microsoft Press.)
I hate to sound like I'm advertising for them, but they do have worthwile offers - if you have the pBook, register it and get a cheap upgrade to eBook. Worst case, you'll pay half the price to get the current revision if all you have is an older pBook version. (You need to have an account with them, though.)
There's no direct link to the current eBook deal of the day, so I'll link to their main page:
http://oreilly.com/
There is an RSS feed, though. (Note: I never tried to buy a previous book of the day, don't know if that would still work.)
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BTW, just my observation, but some of the books can be quite slow (on the Kobo WiFi), it helps to send them through Calibre once. (ePub->ePub, it does something with the CSS, it does look a bit different, but its much faster afterwards.)