Great news: all issues prior to the current one are now publicly available for download as PDF. That's a treasure of now 39 years of TeX articles.
Only the access to the current issue is restricted and is for TUG members only. Until earlier this year, issues became publicly available one year after publication. That was relaxed after a successful open access survey.
Here you can get access and further information:
The TUGboat is available in print too. The printed issues are mailed to regular TUG members.
The TUGboat as well as all TUG activities are for the benefit of the entire TeX community. The TUGboat is funded by member support. So please consider joining TUG. That way you can support TeX, and you get some benefits as well.