I quit my firm this year in August. My former boss didn't take it well and now he ignores my requests for letters of reference. What can I do? I really need those letters of reference. Is there a law obliging employers to provide letters of reference? Thanks a bunch!

Yes, there is such law. Applicable is paragraph 630 of BGB, also the paragraph 109 of the GewO. You should request "qualifiziertes Zeugnis". If you however have worked for shorter time in the company, then you can get only "einfaches Zeugnis". You should explicitely request it by letter. If employer is avoiding to give you letter of reference, it is even possible to get the compensation for damages. There is lot of information about this topic on the internet, but in german. Anyway I think that all this does not help you much. Find a lawyer and he will do this for you.