Apple made a splash recently with its iBooks Author textbook publishing app. I have not tested the app, but it looks quite nice. It is possible to create and publish open source textbooks using this platform. The major drawbacks are that authors must rely on Apple to distribute their books and that readers must own Apple hardware (iPad) to read the books.
There are some fantastic technologies built into the iBooks Author app (e.g. nteractive 3d displays, embedded video). These are technologies that I think will soon be available in more open platforms. In particular, I think that HTML5 will soon be able to match these capabilities and with the distinct advantage of being viewable on nearly all computing platforms (e.g. windows, mac, linux).
According to this discussion, typesetting of mathematical formulae is not easy in the new iBooks Author app. For mathematical and scientific publishing, this is a serious deficiency. I suspect HTML5 combined with something like MathJax will be an excellent alternative to iBooks Author for open source publishers for at least the near term.
Here's hoping that either Apple bases this tool on open standards and provides open distribution options (hah!) or that some tool that leverages the open tools HTML5 and MathJax becomes a viable publication platform. I'm optimistic that it will be the latter as most forward thinking authors and publishers are not going to want to be locked into the Apple ecosystem or maintain their texts in completely separate publication systems.