Your content is not 'public domain', it is your copyright material. 'Public domain' content is not something that you should worry about here.
The content you own can be used in whatever way you wish. Bringing it together in an eBook on whatever platform (Kindle, iBookstore, Nook etc) is perfectly acceptable. My instinct would be to link each of the articles you have already written to the eBook, stating that each article is a free chapter from the book, as an example of the total work.
It would be sensible to 'fill in the gaps' and expand a little without adding waffle, but not necessary.
Be aware that it is not as easy as it may at first look to create an eBook. Read whatever you can on the subject before trying to publish. iBookstore has a notorious number of stumbling blocks, and Kindle is becoming a little saturated (a lot of competition).
In the end it is down to whether your content has an audience on a particular platform. For instance iBookstore has a much greater proportion of male customers than other eBookstores. If your subject is geared towards that marketplace, then it is worth committing the effort there, but be aware you may have to submit the eBook several times before it passes the stringent formatting rules.
Have you thought of self-publishing and adding the eBook to upload sites? You can keep all of the sales price to yourself then.