iBooks Author added very basic support for LaTeX commands in version 2.0 (October 2012). It’s really converting a small collection of LaTeX commands to MathML using blahtex [sic], and rendering them with WebKit on the iPad. In my brief experiments, it appears to be restricted to inserting inline equations in text boxes.
There’s an Apple Support Base article, iBooks Author: About LaTeX and MathML Support, which describes exactly the commands which iBooks Author can use. It’s a pretty short list.
As far as I know, there is no way to add packages to an iBooks Author file, since it’s not really full LaTeX, and it can’t go looking for extra packages. It’s primarily using WebKit and HTML for the backend, not TeX.
Some other notes from my (admittedly brief) usage:
It doesn’t use proper TeX fonts. I’ve not tried looking about an iBook generated this way on the iPad, but the equation in the Mac preview only uses Lucida Grande (the system default font on OS X). Trying common fonts such as Helvetica, Arial and Times leaves the equation resolutely in Lucida Grande, as do trying the Computer Modern fonts. This may be different on the iPad.
A side effect of this is that the formatting looks really weird. Here’s an example:
There’s a world of difference between that, and what a TeX compiler can draw, I think.
I looked at iBooks Author when they announced LaTeX support last year. I write my lecture notes electronically, and being able to use something like iBooks looked really interesting. I tried using it for my notes, but it broke down very quickly. It doesn’t support very many advanced commands, and I just went back to using PDFs and a PDF annotation app.
If your needs are simple, then iBooks Author may work for you. But for any sort of moderately complicated document, I think you’re better off using LaTeX.
I'll end by noting that Apple's record of updating their Mac software has been poor recently. iBooks Author hasn't been updated in nine months, and I don’t think adding full TeX support is on Apple’s priority list. Given further that iBooks is a properietary file format, I wouldn’t recommend using it for maths if you can avoid it.