All excellent recommendations! I'm not familiar with Adler's book, but I've heard good things about it. Personally, I use Kennan's "Technique of Orchestration".
http://www.amazon.com/Technique-Orchestration-Recording-Package-6th/dp/0130771619/ref=sr_1_3?s=books&ie=UTF8&qid=1288896005&sr=1-3
It's huge, the score and audio examples are great, but it's even more pricey. Take a look through the comments of reviews of both, and see what you think. However, for a quick, easy to read reference, I recommend the Essential Dictionary of Orchestration :
http://www.amazon.com/Essential-Dictionary-Orchestration/dp/0739000217/ref=pd_sim_b_1
It's quite small (I like to take it on long journeys), to the point, and while it does cover a lot of topics a few descriptions are on the vague side.
You should have waited on buying the Rimsky-Korsakov though; there's a full, free and updated (with interactive score and audio examples!) available online. Several other great tutorials are available from the same site:
http://www.northernsounds.com/forum/forumdisplay.php/77-Principles-of-Orchestration-On-line
Lastly, "Acoustic and MIDI orchestration for the contemporary composer" is a good starting point for beginners, or a companion. It contains a lot of useful and good information and techniques and how-to, starting with the traditional instruments and then moving onto sequencing them, but is of limited depth.
http://www.amazon.com/Acoustic-MIDI-Orchestration-Contemporary-Composer/dp/0240520211/ref=sr_1_1?ie=UTF8&s=books&qid=1288896496&sr=1-1