Blatter is absolutely first-rate. It is the essential composer's reference book.
It includes practical information about all the instruments of the orchestra, all the voices (both trained and choir), and a selection of less familiar instruments.
The book is organised into choirs (strings, brass, wind etc). The data include varieties of instruments available; their playable ranges (professional, good player and student); how to notate for them; idiomatic usage; trills available; transpositions; weaknesses and strengths; agility or the lack of it; special techniques (bowing for strings, breathing for wind, etc); and short musical examples of typical writing for the instrument (very useful - unlike some other books). Modern innovations are added at the end of each section, with guidance on how many players are likely to be able to do them!
There are also sections on orchestrating for ensembles which are perhaps more of a 'read once' than a 'consult every day', but which give a good outline of what should be in the composer's mind. There's a useful index of notating marks, and appendices which include fingering charts, harmonic series - even the International Phonetic Alphabet.
You may need some additional information when writing for specialist instruments such as harp, recorder, guitar or organ (although Blatter covers them in outline), and irrespective of instrument, nothing beats a conversation with a good player.
But for general every day use, Blatter is absolutely essential. If you are looking for just one reference book, this is it. I use it all the time - while Walter Piston, Rimsky-Korsakov and the rest gather dust.
Four hundred pages of pure gold. Not cheap - but worth every penny.