I recently noticed that David Vining has published a book entitled "Trombone Intonation Mastery." I don't have direct experience with this book, but have another of Prof. Vinings books (Range Songs) and have found it useful. I suspect that, given Prof. Vining's reputation, this would be a very useful book also. (In fact, I might buy a copy for myself.)
It is available from Amazon, directly from his publisher Mountain Peak Music (www.mountainpeakmusic.com) or maybe your local music store could get it.
Hope that you have an enjoyable experience playing trombone (but also hope that you keep up your euphonium playing.