I’ve been hard at work on various LaTeX3 questions (more on which in another post), and little things about TeX come up all of the time. These are often rather technical, and so what I need is a good reference work which includes all of the detail. Now, The TeXbook is an obvious place to look, but it’s not available electronically. I do a lot of my coding when on the move (on the train, at work, various places around the house), and so carrying a book about is not always so easy. There’s also a lot more to The TeXbook than just a reference work, which doesn’t always make it quite so easy to quickly look up a particular primitive.
On the other hand, TeX by Topic is available electronically (and for free), and is a focussed reference work. As well as being able to download it from the author’s website, it’s probably installed with your TeX system:
at the command line should open it up. I always find the content in TeX by Topic to be excellent: enough to help me out, but not too much that I get lost. For a TeX programmer, I think there is no better resource for those fiddly questions. Being available for free is of course an added bonus, although I would strongly encourage making a contribution to the author (I certainly have). For those people who want a printed copy, TeX by Topic is out of print but is available from Lulu. So all round it’s an excellent choice: great work!