1. If you require login before giving these users access to the download section you can use their Membership data - MembershipUser.Email Property
2. If you do not require login you can create an standard ASP.NET HttpHandler and if someone makes are request to your library send it to a custom email form. The user should type its email there and you can persist it in a custom table by using LINQ or Open Access ORM. You can easily query this table and check if the email exists so you can add a new record or just proceed with a download. To avoid multiple redirection for the same user you can create a cookie with maximum expiration date which will allow the same user to download files multiple times without going to your form which will be not necessary.
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