"I have a quick general question, which is probably incredibly basic, but I don't know how to go about obtaining the answer elsewhere: How does one find out what the astrological conditions are or will be for a certain date and time? I know there must be a recurring pattern, but it's not something I've come across in my studies. Is there a introductory-level book which might be able to assist me?"

positions of the planets are listed in the tables called Ephemeris.
(every astrology software has them, so you can calculate a chart using software and never look at them; but it's better to have them outside of a programe, so you can follow the planets' motion easier)

maybe "Astrology for beginners" could help - to just get to know what-is-what.
and here's some encyclopedic astrology info:
www.4shared.com/document/lEekXwzQ/James_R_Lewis_-The_Astrology.html
www.4shared.com/document/TvBAhJ0V/Encyclopedia_of_Astrology_-_Ni.html

don't know of a introductory book with the western Sidereal approach

"We will begin working probably at noon on May, 1 in Tupelo, MS. I have Frater Arsihsis' American Sidereal Ephemeris but he's out of town & I have no idea how to read this thing! "

A little late, but...the 1st isn't overly favorable for working, as the Moon is in Debility and strongly squared to both Saturn and Jupiter. But then again, many important things have been done under unfavorable skies.

"I have a dilemma. I am planning an entheogenic working framed round the LXV formula for Beltane. I am not very astrologically inclined & I'm wondering if the stars will be in favor (I'm not even sure what constitutes favorable stars) for the working. Would anyone mind offering some thoughts or suggestions?"

What is the date and location of the working?

(It's nice to see people correlating their workings with the heavens. This is a much neglected aspect of the Art, IMO.)

" **an entheogenic working framed round the LXV formula **for Beltane"

I'm not familiar with this kind of working, but if you explain it a bit, maybe I could help with the astrological data
...............

edit: if you ment LVX formula, then time when the Sun is rising (for the place where you abode) seems good enough. (for example, half an hour before and half an hour after the exact minute of rising, if you plan it to lenght for an hour or so... or just begin at exact time when the Sun is ascending)

"The space-marks are aright for the Working of this night!"