The Speil ballade for cornet, among those works, found in the Library of Congress American Memory Collection, seems (to my non-performer's eye) worth a look-over. What I can gather of his biography is interesting...
The Saint-Saens Concert-piece - which I remember fondly because a horn-playing family member used to practice it - is not the very most sung of his works (neither is Weber's concertante work for the horn, opus 45, memory serves) and for myself I think well of both- though again for myself best of these is the Schumann (though there are fortunately a number of very good horn works, and if embarrassingly for me the first time I saw the Brahms horn trio mentioned (in a concert advertisement at Interlochen, when I was working there) I thought it was a trio of horns- later I learned there are quite a few such pieces, by Reicha for instance- it is deservedly "sung" and singing, too (and therefore tangential to the thread- though it probably inspired a number of responses, besides the Ligeti Hommage à Brahms obviously and a few almost definitely- I think those by Jenner and Herzogenberg, almost definitely...)
There's also a few horn and strings quintets besides those by Mozart and Reicha - a late-Romantic one by Emil Kreuz, for instance (and the horn, piano and strings one by Felix Draeseke.) Robert Kahn wrote a serenade for oboe, horn and piano that was published in 1923 (so I haven't seen it - copyrights stuffs) but I suspect is good... (recorded on Chandos in 2002 in one of its alternate forms, with viola for horn.)