Make sure you have a page on your website solely focusing on those free offerings. This is going to be a key landing zone that you want to drive people to. Customize it so it's all about the FREE nature of things. Sure, you might also have other regular book pages that promote those same works, but this FREE page is for its own special purpose. The word "free" draws in lots of readers. Take advantage of that.
Here is my page on my 20 free ebooks -
Lisa Shea's 20 Free Ebooks
I use that in my online marketing. But how about if you're going to a book fair or show? What do you bring with you?
Free Book Flyer
In essence you create that same functionality on a piece of paper. You want to feature each cover so the reader remembers what the cover looks like. Then you want the title and description for each book. You might as well use both sides of the paper, if you're going to be handing this to a person. Otherwise the back side is just wasted! Feature a blurb for the book, an author bio, and anything else which might help make the sale.
Remember, the user is going to have to go do extra work to get their hands on these books. You want to make it as simple as humanly possible for them. Don't give them complicated URLs to type in. Most users know how to use the "search" feature on their Kindle or their Ipad or so on. Give them the title and your author name. The cover image helps ensure they don't click on the wrong book by accident.
Here are images of my two-sided flyer I take to book festivals. It promotes my 20 free ebooks. Click on either one to get a larger version.
This is the kind of flyer even the most jaded book festival visitor will tend to pick up. They get innundated with marketing material which says "spend money on me! spend money on me!" but this one says "here's a free treat." They will keep that and remember that. And they're very likely to download it, if you make that process easy for them. Which then means they get drawn into your funnel and hopefully keep reading your other material!
Lisa Shea Free Ebooks
Lisa Shea Full Library of Published Books
Getting Your Book Published
Writing Tips and Online Books
Lisa Shea Medieval Romance Novels
Online Literary Magazines
Lisa Shea's Homepage