The (rare) dictionaries and grammar books for German dialects exist for reasons of a) research b) entertainment. They are not used for teaching classes (isolated exceptions not considered).
You will have to search very long to even find classes to teach you dialect. Yes, there are some, e.g. I know that municipal adult education centres (Volkshochschulen) in Northern Germany sometimes offer Plattdeutsch classes, and i would suppose that you might find something like this also in other dialect-loving regions. But this is again rather for entertainment and to some extent also for preserving cultural heritage.
If you ever consider attending some course like this, be aware that they will teach it in a contrastive way, i.e. based on the standard language (Hochdeutsch). So, standard German is the first thing to learn.
Being able to understand and even actively use dialect may, however, be really useful when you happen to live in a dialect-prone region and want to socialize with people who live a very local lifestyle: live and work where they were born and raised. They might be actually close to unable to use standard German in oral communication (in writing they use standard German still), or feel very uncomfortable using it, and would be much more open to people speaking to them in their dialect. So, that could be a motivation to actively learn a dialect.
The problem is that these dialects are not standardized (well, they are dialects), meaning that e.g. Swabian in the village of Schweinhausen will be noticably different from Swabian in the village of Äpfingen 20 kilometres away (I just picked some villages, there is nothing special to them). So people will always instantly understand that you are not from their immediate area.