Fill out the form.  Assuming you're in IE (similar steps for other browsers will probably work), right click on the book image in the blue pane and select "Inspect element".  In DOM Explorer, scroll down a little and select the element below the highlighted one (the a tag with the body "Download your free eBook").  In the Styles page on the right, scroll down and locate ".hubspot-form-submit, iframe#download-form".  Uncheck the line for "display: none;"  Back in the browser page, click the newly-visible "Download your free eBook" button.