Improve search engine: be tollerant with typo and plural
Searching for "mathématique pour l'économie knut" do not show any good result.
Searching for "mathématiques pour l'économie knut" show the book.
It's definitely possible to configure solr to understand those kinds of typos and propose the result. This is specially easy because it's a simple plural form. But more generally small typo should not be a problem for the search engine.
There is a good blog post https://jolicode.com/blog/construire-un-bon-analyzer-francais-pour-elasticsearch showing some of the technique (for French and in elasticsearch but the same approach is applicable on solr).
You can also use phonetic matching https://www.elastic.co/guide/en/elasticsearch/guide/current/phonetic-matching.html
You should really ask (hire) @damienalexandre to look into your configuration.