Keep your eBook projects simple. Avoid complex layouts and formatting, only use simple tables and don't use invisible topics. Only a..z, A..Z, 0..9 and _ are permitted in topic IDs in eBooks!
The ePUB 3.0 format introduced in Help+Manual 9 is fully backwards compatible with ePUB 2.0. Even older ePUB readers can open and display an ePUB 3.0 eBook. There is no break in versions or any minimum requirement. However, ePUB 3.0 offers more freedom in page design.
Help+Manual now has additional formatting settings for ePUB. This includes an editable Table of Contents template and an editable HTML page template. These are similar to the corresponding templates in WebHelp, but you should be aware that the formatting options in eBooks are still quite limited compared to HTML web pages. Avoid complex layout and formatting. Large and complex tables and tables containing images should be considered to be toxic, because they will almost always destroy your layout.