Is there a LaTeX widget for iBooks Author?
The widgets available at the moment are listed here. There seems to be some confusion in the question though. Widgets are the name given to dynamic elements, such as audio or video, and other interactive elements. LaTeX does not really fit into that broad definition. If you need LaTeX for equations then the best option it to write equations or other self-contained snippets in LaTeXiT and use the generated pdf files in iBooks Author.
MathJax 2.0 supports an SVG output and equation numbering. The SVG output is noted to be compatible with iBooks, so this would seem to be an ideal solution.
As of version 2.0, iBooks Author now supports both LaTeX and MathML equations.
Althogh iBooks Author offers the link to MathType, it looks MathType equations are not in PDF format.
LaTeXit or MathMagic equation editor supports PDF format, with the font embedding feature. You can also copy & paste the PDF equation back to the original equation editor to reedit if necessary. The quality of PDF equations looks much better and sharper, especially for the lines, root/radical signs, and small script/accent characters.
It turns out that using Insert... command in iBA is not faster than inserting via Drag&Drop or Copy&Paste, if you have to deal with many equations and symbols. Or, it would be better if there is an equation button on the toolbar.