How do I create a "More like this" block?
Here is the recipe for search_api_solr as per the OP. Note: both apachesolr and search_api_solr leverage Solr's builtin more like this feature. It is, therefore, not available to normal search.


Enable some modules: search_api, search_api_solr, search_api_views
Create a Search server of the Solr class.
Create a Search index (or use the Default node index) and point your Solr search engine at it.
Create a new view on your search index NOT on Search index. 
eg: 

Show *Default node index* sorted by *unsorted*
Continue and edit.
Reveal Advanced and Add a Contextual filter.
Filter on Search or scroll down to find Search: More like this.
Under When the filter is not available select Provide default argument: Content ID from URL.
Configure by selecting the Fields for Similarity that you want to determine likeness on.


http://drupal.org/node/1250304#comment-4870278