If you are reasonably familiar with the principles underlying Tarot reading then you have a fair basis to start Horary. The idea of a querent (person who asks a question), the question asked and the Astrologer/Tarot reader who answers the question underlie both.
Horary Astrology can trace its roots back to classical Greece and many of the 'rules' can be traced back centuries. A good starting point is Anthony Louis' book on Horary. Louis has also written books on Tarot so he is a good starting point for the taroist wanting to learn about this part of Astrology.
Don't bother with anything on natal Astrology. Modern natal Astrology is as different from Horary as chalk and cheese. The key is to understand what the question is (sometimes the querent will need some help to express this) and then identifying the house (or houses) that need to be examined. This is by far and away the main thing.
The question is answered by examining the rulers of that house (or houses) and assessing their condition. There are some rules for this assessment and these of course need to be learned.
When more than one house is involved there is the second issue of seeing if the rulers (usually two of them) will form an aspect - no aspect then no outcome.
A good website which features horary is Deb Houlding's Skyscript
http://www.skyscript.co.uk/
It's also a great resource for Traditional Astrology and you will find answers to many of the questions you raise as you read Louis. There's also here of course as there's a number of people interested in horary.
