IMHO, the best book out there is "The Study Of Orchestration" by Samuel Adler. It is sort-of in text book form; you won't be reading a novel-type book, but at the same time you won't feel like you are sitting in a boring classroom. Comes with optional CDs and workbook.
In my humble opinion, I think dpc is on the right. For example, you can read a lot about 'The Rite Of Spring', some people explain the score from an atonal perspective (pitch-class set analysis), others from polytonal harmony, but when you have the full score you can obtain your own conclusions.
Ravel, Debussy, and Schoenberg.... Those cats work's aren't modern - they're pushing 80 to 100 years old now!
There are several books you might investigate if you're looking for orchestration books discussing more recent composers.
1. Read Gardner's "Orchestral Combinations" lists examples from Mozart to Corigliano. Late 19th through 20th century composers are well represented in this text. Unfortunate with this book is that he references page/rehearsal/measure numbers in the scores and you need to go find the score to see what he is talking about. This book deals with combinations - doublings (and octaves) and the resultant timbres.
2. Read Gardner's "Thesaurus of Orchestral Devices" is what the title states. Mr. Gardner also has other titles discussing specifically more modern techniques a la Penderecki, Lutoslawski, Xenakis, and Crumb.
3. Andrew Stiller's "Handbook of Instrumentation" is not an orchestration book per se. It heavily references works from the last 50 years or so.
4. Often biographies and single composer analysis books discuss at length the orchestration of the composer's works. There is for instance and exellent book on La mer that really breaks down the composition and orchestration. The name escapes me currently, but it might be something like "La mer An analysis"...
5. Google the works or composers. Often there are dissertations posted discussing the composer's you listed. It's pretty fashionable now to discuss things like the impact of timbre on a work in graduate studies.
NPC Imaging carries most of these books as well as Dover scores and CD-ROMs.
Orchestration is taking a piece and faithfully, though hopefully creatively, expanding that piece for orchestra. Arranging can also mean this but implies a level of creativity, maybe adding or 'improving' harmonisation adding countermelodies and all the tricks.
If you took a classical piano piece and scored it for orchestra you'd be orchestrating, if you took the top line and chords of a popular song from a fake book (requiring creative input on your behalf) and scored it for orchestra you'd be arranging.
I've probably confused everyone now!!
The comment about taking a recording of a favourite piece and sitting down with the miniature score provoked memories of being a student - there is SO much to learn when you listen and look, how it was done is always fascinating. A tip one of my teachers suggested is also good, sometimes take the time to follow just one line through the score - be the first horn for a bit, say - the dynamic of 'being' that player gives you a real insight into the music too.