The answer demands fine judgement in walking the thin line between giving away too much and too little. Content is King on the Internet. A Website, eZine or even simple eMail message lacking content appears bare, shallow and worthless. You've doubtless come across them in your Web-wanderings. Just as surely, you've backed out of them to go elsewhere, where there is more 'bang for your buck'.
Information is the fuel of the Internet economy.
The Website or newsletter or eMail message that provides them with the information they seek is perceived as valuable. The rest are grouped as 'trash'. So it is obvious that you, as Webmaster, must give your readers content. Not in the long run, anyway. On the other hand, without offering some part of it to your readers you can't build traffic to your site or credibility.