Really great tool that makes a lot of tasks singifiacntly easier than other programs e.g., layout, equations, formatting, publishing, adding interactive media. One big drawback that inexplicably persists is the lack of the standards iBooks page curl effect. Instead the only option is for the pages to scroll by like boards. Not a super important drawback, but a strange and disappointing one. More importantly, there’s a bug with interactive images that have a lot of annotations in them: the detail view will sometimes end up below the title-only view of another annotation. There seem to be no z-axis arranging options for the annotation elements, though there shouldn’t need to be because there’s really only one expected behavior (that the current detail view should always be top of the z-stack).
Another thing that would be nice to see is the ability to properly zoom with the 3D widget. Going full screen is great, but you should be able to zoom, just like using OSX Preview on .dae files.
If the above were fixed/added, this app would definitely be 5 stars.
As for the issues others are having with iBooks running in El Capitan — I’m using OSX 10.11.2 and iBooks Author 2.4 and have no such issues.