One book that has been in my library for many years is The Business: The Essential Guide to Starting and Surviving as a Professional Horn Player. Compiled and edited by well known British freelancer and composer Paul Pritchard, The Business is full of practical advice on many facets of living and working as a professional horn player.  Looking at the table of contents, the authors are a who’s who list of top British horn players.

Chapter 1: Your First Professional Symphonic Date, by Jeffrey Bryant
Chapter 2: Opera and Ballet, by Julian Baker
Chapter 3: The Horn in the Studio, by John Pigneguy
Chapter 4: Solo Performance and Chamber Music, by Frank Lloyd
Chapter 5: General Freelance Work, by Paul Pritchard

And although the authors are British, their advice is for the most part applicable to the same professional situations in the United States.  In re-reading through the book I noticed that intonation was a topic which kept coming up.  Three of the five chapters have entire sections devoted solely to the discussion of tuning and intonation, so obviously it is an important area to consider.  Although impressive technique and range can open many doors professionally, your ability to blend and play in tune with other players will keep the opportunities coming.  Here are a few of my favorite passages from The Business on the topic of intonation.

From Jeffrey Bryant in “Your First Professional Symphonic Date”

Perhaps the most difficult aspect of playing second [horn] is intonation.  You always have to tune to the 1st horn, even if you believe him to be way out of tune.  This is difficult because it means that you will not be in tune with other instruments that are playing in unison or octaves with you.  When you are put in this impossible situation, my advice is to use a light to heavy vibrato, depending on the severity of the problem.  You will be amazed at how effective this can be.  Never be dogmatic about intonation, always be sensitive and try to adjust to fluctuating pitch.  Don’t try to impose your pitch on a tricky passage by playing louder.  Play quietly and try to merge into the overall sound. p. 11

From Julian Baker in “Opera and Ballet”

Tuning in the pit has its own special problems.  While of course one must be in tune with the principal and other members of the section, it is worthwhile keeping your ears open for general intonation in the pit, by focusing your attention on a larger section of the orchestra preferably not too close to your own.  The ‘cellos’, for example play a similar range to the horn and are good indicators of pitch.  The pitch of all orchestral instruments varies according to the temperature and humidity, which can reach uncomfortable extremes.  Under these conditions, brass instruments will naturally rise in pitch, woodwind also; however the strings will tend to go the other way.  In anticipation of this they sometimes compensate by adjusting the tension of the strings, giving a brighter edge to the sound.  Over a long opera or ballet, this can sometimes result in different blocks of intonation dispersed throughout the orchestra.  This can affect the tuning within each section, as each player tries to cope with the problem in his or her own way.  Although this is a difficult situation, with a little humility and an awareness of the principles of tuning, it can be overcome. p. 23

And from Frank Lloyd in “Solo Performance and Chamber Music”

Tuning however, can still continue to be a problem, in fact it can be one of the most frustrating aspects of chamber music, and a constant cause for concern.  In my opinion, it is the one factor that separates the really top class ensembles from the rest.  Listen to the best groups, either instrumental or vocal, and you will immediately be aware of their impeccable tuning.  No instrument is capable of being built completely in tune, and those fixed instruments such as the piano have to be tuned to a tempered scale, which is in fact a compromise, and the closest that can be achieved but which is far from perfect…In many cases, it will not be sufficient just to play the notes where they normally are, – this might not work in respect of the overall tuning.  At times you will have to bend notes a long way from where they normally sit, and often use alternative fingerings to achieve the desired pitch.  Much depends on the position of the note within the chord.  Experience of the repertoire will enable you to play in tune within the harmony, and also when you have the lead line. p. 68-69

The rest of the book has lots of other great material, and although it may be a little tough to track down, The Business is a great resource to have.