This is "A Saint with a Book" by Michele Giambono, painted in the Venetian style. The artist's use of gold and decorative patterns is quite striking - look at the details around his halo and cloak. His book has a dark green binding and gilded fore-edges, adding to the sense of luxury and importance.
We're not entirely sure who this saint is, but he might be Saint Mark, based on traditional depictions of him. The wavy line around the figure shows where the original frame would have been - it was part of a larger altarpiece that's now scattered among collections worldwide.
A Saint with a Book hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.