Final Status October 2013 --- eBook is now available with a special promotion price until the end of the year
I'm happy to announce that there is finally an eBook version of "The LaTeX Companion, 2ed" available. It was a rather challenging exercise due to the many examples in the book and the need to reproduce the example output faithfully on that media. Originally we thought that anything other than plain PDF is not going to be possible; however as it turned out both ePub and mobi (Kindle) came out surprisingly well.
The eBook will be sold through various channels, however, the best deal is probably going to be through InformIT, as the publisher is selling the eBook as a bundle with all three formats included (PDF, ePub, mobi) and no DRM applied (!) only watermarked to kindly remind readers that this is not a work given away for free.
In addition there will be a special promotion price for the bundle valid until December 31, 2013 where it is offered at $14.99.
To make use of this special offer, you have to buy it from the publisher at InformIT
and apply the following promotion code during checkout processing:
LATEXT2013 (Case sensitive! and notice that it is "LATEXT" not "LATEX" for some reason)
In 2014 the official price will be $23.99 at InformIT which I still think is a good price for all three formats. Be careful if you see the book from other resellers, they may apply their own DRM which, in my opinion, makes an eBook nearly unusable.
Enjoy -- Frank
Older info (kept for historians :-)
Status in June 2013
Beside the fact that TLC2 is indeed available on Safari (subscription) as mentioned by Joseph in his answer both as pdf and as html (urg, but in fact it could be worse, so after seeing that we decided to let this be) there are news on this front:
I worked with the publisher to make a properly cross-linked pdf (the safari one was just a simple pdf, searchable but without cross-refs) and we are currently looking at whether or not providing epub would be possible too (if the quality is not too bad we may ok this too). The pdf version (no DRM, only watermarked) is already available at InformIT
and if the epub works out it will soon be added too. It may also be available from some resellers but be careful there you may end up with Adobe DRM or similar nasties.
I would like to make it available cheap for those who already have a hard-copy of the book, but given that there is not a good scheme how to arrange for this we will probably run a limited promotion (with a bargaining price) at some point in the near future which we will announce in the TeX world, so stay tuned.
However, if I may say so, an awful lot of work went into this book and its 1200 pages and I believe its normal price is actually worth it whether as hardcover or ebook so if you can afford it and you want to give something back to the authors ...
Status in August 2012
The second edition of the LaTeX Companion is not available as an ebook (at least not legally, give or take some scanned copies that float around on the internet). For this edition this status is very unlikely to change. (I'm the author, so I'm not just guessing).