In horary astrology the time of a vital question determines the time of the horoscope. In other words, the "birth" of the question occurs when the client asks an important question, the astrologer understands the question and a chart is constructed.  Usually the question is specific relating to the outcome of a business venture, the location of a missing object, or the possibility of the return of a lost love.
 

A few years ago, I took an excellent class in Horary from astrology software programmer Allan Edwall.  In eight weeks we intensely studied the rules of horary but it was just a beginning to this complex art.  He recommended books which seem to be out of print now but were based, as much of modern horary is, on the works of 17th century astrologer William Lilly, especially his book Christian Astrology (found on the  JustUs & Associates site, which includes a horary certification course).

My favorite book on Horary astrology is one by Anthony Louis that is currently out of print.  He has a new one called Horary Astrology Plain & Simple : Fast & Accurate Answers to Real World Questions which was reviewed well by the customers on Amazon.

Or try these:

Marion March and Joan McEvers cover horary astrology in the sixth book of their series:
The Only Way to Learn Horary and Electional Astrology.

Horary Astrology Revisited by Olivia Barclay is a well regarded horary book (Ms Barclay also has a certification program for horary astrologers).

Horary Astrology by Marc Edmund Jones.  This book, along with Olivia Barclay's is on my shelf to read "one of these days."  I love horary astrology but it's much easier to learn from a teacher than a book.