InDesign obfuscates the fonts it embeds into the document. But this is not a guarantee or an automatic permission that you are allowed by the the font creator to do so. Font files can have a flag integrated which denies Software such as InDesign or Illustrator to embed or package the font. This would have nothing to do with InDesign in this case but with the font creator. In addition, please note that font vendors/creators may have special license therms for ePub embedding (as obfuscated or not protected, depends). Looking at the Slimbach font, I have found this info: http://www.linotype.com/799/ITCSlimbach-family.html (see ePub drawer) as well as
If you own this ePub license of the font, I suppose there is something wrong with the font file itself, so InDesign could not embed it. It is an OpenType or TrueType, right? Type1 or Bitmap fonts can't be embedded.
And also note: Obfuscated fonts do not work in Kindle documents. So if a vendor writes in his therms that you are only allowed to embed obfuscated fonts, you don't need to embed them for Kindle as it can't read when obfuscated and you are not allowed to embed when not obfuscated.
Thank you Apel! It appears I have purchased the desktop font. I'll be purchasing the ePub version and hopefully that will work.
Thanks again for the information regarding Kindle, I'll also keep that in mind.