I'm a guitarist, and I struggle when writing for horns. Any advice?
Along with reading instrumentation textbooks, memorizing ranges and understanding how the instrument behaves throughout its range, talk to musicians who play these instruments. Rhythm section members often write music that works best for their own instrument (guitar or piano, for example). Ask a wind-playing friend to play your music and ask questions. Was it easy to play? Is it too high? Too low? This is an invaluable (and free!) resource.
How can I get started with a decent jazz collection?
The Smithsonian, together with Folkways Records, has released a new version of the seminal Smithsonian Collection of Classic Jazz, re-titled as Jazz: The Smithsonian Anthology. You can find it here.
What is a good jazz history book?
There are so many out there now. However, my favorite is Ted Gioia's The History of Jazz. Many college jazz history courses are now using Jazz by Scott DeVeaux and Gary Giddins. The Gunther Schuller books, Early Jazz and The Swing Era are fantastic. Lewis Porter's book Jazz: From Its Origins to the Present is a good overview of jazz history. An extremely user-friendly book is Mark Gridley's Jazz Styles. And don't forget the Ken Burns PBS series on jazz.
When is it appropriate to use a spread-out "open" voicing?
Your music will sound the most "agile" when your parts are moving in close harmony, as in a sax soli. However, you'll notice that spreading out the harmony allows the listener to hear each line more clearly. If that is your goal, go for it--just be careful of range.
When I transcribe a score, I really struggle hearing the inside voices. Any advice?
Make sure you have the chord changes in front of you at all times. If you can't hear a specific voice, use your theory knowledge to help you determine what the note would be. For example, let's say that you've transposing horn parts and the chord of the moment is an F7. You can't tell if the tenor sax is playing a C or a C#. Your theory knowledge will lead you to the conclusion that the note must be a C, as that is the fifth of the chord.
How specific should my rhythm section parts be?
A good rule of thumb is "less is more." Often the rhythm section members will come up with the best solution by simply reading slashes and chord symbols - that's what they are trained to do. Obviously, if something very specific must be played (and you want it played that way every time the chart is performed) then you must notate it.