A great resource to learn more about inbound marketing is the online community Inbound.org. One of the members recently posted the topic/question, "How to offer clients a downloadable eBooks/PDFs?" - this is a great question.
A complete inbound marketing strategy includes creating content that attracts new and relevant visitors to your website, but it doesn't end there. Converting your traffic into contacts is a key next step, and offering premium content is a cornerstone of conversion. You need a way to for someone to easily give you their email address to get access to that content.
Why do you need their email address? The truth is that you don't, you could just give them the ebook for free with an immediate download, but I wouldn't recommend it. Ultimately you want to help this person determine if you can help them solve their problem they are researching. To do that you need to build a relationship with them, so you are trusted when it comes to that decision time. Relationships and trust happen over time, not in one download. They'll probably need to hear from you at multiple points in their buying journey which will probably happen via email.
Back to content and delivery. I'm including my answer below to help explain some easy ways to capture and email and deliver that content. Some are complete solutions like HubSpot, others are simple like Contact Form 7. Whatever you choose will help you take the next step with your inbound marketing journey.
Hey Rachel - If I were in your shoes, I'd start by making a decision on if you want them to download the ebook immediately OR you want to deliver it via email. If you want them to download it immediately, then @patrickcoombe gave some great options.
With our clients, they tend to be downloading one of our offers fairly early in their discovery so we want to capture their email address and we then deliver a link to download the book to their email, which helps set the expectation that they'll hear from us via email again. (We normally use HubSpot for what you're asking about).
Your options to do this are pretty wide open. We've used a few different tools in the past to accomplish this in Wordpress for simple or limited delivery.
A couple simple tools:
1) Use the plugin Contact Form 7 to capture their email and send an auto-responder email as a notification/link to download.
2) If you use Mailchimp and want to use them to store and send your email, the Mailchimp for WP plugin is an easy way to collect the email address and send the email for download without requiring a double opt in for the client. (Sometimes this doesn't send immediately, YMMV)
More Complicated
3) If you're looking for more of tool set, you can try the tools over at Inbound Now, they've got a plugin for Landing Pages, CTA's and Leads. To be honest, I think this set of plugins are powerful but frustrating to implement quickly. The settings are circuitous and redundant and their documentation is lacking. However, depending on how many ebooks you want to set up and how much information you want to learn about a lead, they can be worth it.