Hi @JohnNiggl
Pillar pages can be any type of content. What matters is that the content meets the requirements of a pillar page, i.e. covering a broad, relevant topic comprehensively, not what format it was in originally.
For some, un-gating an ebook is the easiest way to quickly create a pillar page. This obviously replaces a landing page and some like to retain the conversion capability, meaning keeping it as a landing page. But it is not a necessity.
Your pillar page can come from anywhere - it can be an existing web page, an un-gated ebook, repurposed blog post or completely original content.
Hope this helps.