Denis Simonov is the author of Firebird documentation, seasoned developer and DBA.
Denis Simonov sells the following books and courses in FirebirdSQL shop:
1 . Book: Detailed new features of Firebird 5, 2nd edition (purchase: €16.99 / US$19.99)
2. Book: Coming soon! Secrets of Firebird Query Optimizer (pre-order will be available soon)