I wrote an article (in german) for the Java Spektrum on our approach of implementing learning to rank algorithms in our Apache Solr product search.
To read the article, go buy the magazine. Its contents are (mostly) covered in a talk Fabian, René and I gave at Haystack Europe in October 2018. You can find the slides here.
If you have any questions regarding the article, the talk or LTR in general, hit us up on Twitter!