Pollen is a publishing system that helps authors create beautiful and functional web-based books. Pollen includes tools for writing, designing, programming, testing, and publishing.
I used Pollen to create my book @link["http://practicaltypography.com"]{Butterick's Practical Typography}. Sure, go take a look. Is it better than the last digital book you encountered? Yes it is. Would you like your book to look like that? If so, keep reading.
Not that you need to be a programmer to use Pollen. On the contrary, the Pollen language is markup-based, so you can write & edit text naturally. But when you want to automate repetitive tasks, add cross-references, or pull in data from other sources, you can access a full programming language from within the text.
That language is Racket. I chose Racket because while the idea for Pollen had been with me for several years, it simply wasn't possible to build it with other languages. So if it's unfamiliar to you, don't panic. It was unfamiliar to me. Once you see what you can do with Pollen & Racket, you may be persuaded. I was.
If the file extension is ``@(format ".~a" world:preproc-source-ext)'', the source is interpreted as [pollen/pre] (``pre'' stands for ``preprocessor'').