In Markua 0.30, you can add index entries to books.
When you add an index entry directive to your manuscript, the index entry will be added to an "Index" section at the end of the book, whenever you generate your book.
The simplest index entry directive looks like this: {i: "Ishmael"}. You would add it to a line in the manuscript like this: Call me Ishmael{i: "Ishmael"}."
In this sample book manuscript, you will find an instance of each different index entry directive in the chapter1.txt file in the manuscript folder.
The section in the Markua specification setting out how index entries work in detail can be found here: http://markua.com/#index-entries-m-.
Finally, here is a link to the Markua manual, which shows you everything you can do when you write a book in plain text on Leanpub.
Good luck!