If you do designs on a regular basis, and can cough up the $$, get Time Saver Standards for Landscape Architecture by Harris & Dines.
I bought my copy for a class about 4-5 years ago and paid $150 for it then. I've referenced it a number of times since then.
For ID purposes, Landscape Plants for Eastern North America by Harrison Flint and Manual of Woody Landscape Plants by Michael Dirr are both good books to have. I have both, though you could get by with just one, the Dirr book is probably cheaper and more entertaining at times.
I find the RS Means book that I have to be somewhat hard to use for landscape installation purposes. It'll get you in the ballpark, but you really need to know what it takes YOU to do the job...
Are you looking for business related, how to build landscapes, how to price, how to market, lawn care, ... It is a broad topic.
Time Saver Standards for Landscape Architecture might be great for you , or useless to you, depending on what you do. It is a reference more for space and layout requirements for larger projects than something beneficial to a residential landscape contractor. It has ADA requirements (ie, handicap ramps), parking and vehicular circulation, sports court dimensions,..., but not how to landscape Mrs. Smith's back yard. Excellent book for those that need it, although it tends to physically fall apart after a while because of its size.