Jiarby raises a good question, and if you don't have the exact answer yet today, I would even suggest you worry about that first, & getting the addresses later.. granted, collecting email addy's from web visitors is a long process, so starting sooner than later makes sense... but once you have some, you need to have thought about what to say to them that will be well received... and speaking from experience, that also takes time..
In fact, I have hundreds of hours into my own over thought & over-developed idea to do something quite like what you speak of... and I've posted about it a few times... but after I got to where I had something of some value, ready to GIVE out to site visitors for FREE... I had also come across many deeper philosophical discussions about spam, about marketing... and about "FREE"...
In the end, I decided something is NOT free if an email address is required in exchange... so, I backed off my original plan. It didn't really die on the vine, & I suppose the link to download the guide may still require an email address.. but I have not promoted the guide, so nobody knows about it except some of my signs101 friends & the rare guy who scours a site, looking at everything...
..what I DID do is make a printed version of my "Sign Planning Guide" to help with sales discussions both in & out of my office. What I also intend to do is a mailing to advertise the website & the FREE guide, at which time I will remove any required email address... BUT... what I will try to do instead is to look like the guy with something useful to say, AND to give away, and to be the kind of knowlegable & generous source of sign related information, so that when I will ALSO have an invitation to subscribe to a sign newsletter, in all my email signatures, near my FREE guide download link, and in my promotional postcards, truck ads etc... THEN, maybe I will start to build up a database entirely from folks who KNOWINGLY opted in to an intentional desire to be kept informed of my marketing advice, promotional offers & perhaps sign related trivia & anecdotes...
and THAT is the demanding challenge that lies ahead... writing (or finding, quoting and attributing the source) relevant content, worthy of my clients subscription, attention & continued loyalty... and THAT is the much harder part of this long process...
but anyway, here is a link (containing other links) to a discussion of a similar concept:
http://www.signs101.com/forums/showthread.php?t=56722&highlight=planning+guide