For great reference and some rare photos, the Theater World series (which stopped publishing a while back) is great because it takes you through each Broadway season in detail: replacements, stand-by's, then goes through each of the Off Broadway companies, national tours and various awards given out. There was a large section on obituaries and major cast member bio's. The Best Plays of (season year) is good because it has 10 shows which stood out, basically a full plot synopsis with important scenes (and in the case of musicals, songs) done in detail.
As a kid, I would scan through my school and local library of the individual scripts of plays and musicals, and when I moved to a bigger city, went nuts with the number of more obscure titles I found. They were hard-cover, not paperbacks, with a great book cover if still in good condition, and often, stills of the original production. Before I started buying souvenir programs, I would scan the pictures, although now through the NYPL and Museum of the City of New York, you can find tons of rare stills on line that were never available before.
One Grecian Ern