I have an old list of about 50 titles (both with and w/o book summaries) that I'm re-tweaking and will blog up in the next week or so. Some titles have already been mentioned by others here, and though I tend toward reference works (the which you too seem to prefer), I include fiction and other titles, some of which aren't Reformed - things like C.S.Lewis, Peretti, Eldridge, Lutzer... And you've likely already worked through library bare basics: a good reference study Bible, a good comprehensive concordance, a good Bible dictionary, a good 1-volume Bible commentary, a good summary of Christian doctrine (Berkhof & the Westminster Standards are excellent), and something on Bible difficulties. There are a lot of peculiar reading path choices after, like some may lean toward systematic theology tomes, Bible book commentaries, things like Kingdom of the Cults and world religions fare, pastor and self-help titles, or other genre.
um... you want a Reformed Baptist slant? I loves me some James White. And, you know, there are at least a horde and a half of worthy books on baptism alone. I like the all-in-one books that offer several views from different perspectives.
And understand, I'm one of those nutty guys who has over 100 types of Bibles alone, but I understand if you want to branch out a bit from there.
And, btw, who wants my library when I die? Because I'm looking for a candidate. Hopefully it may be a while yet, so maybe a young whippersnapper who anticipates some reading time on his hands. Or, OK, I'll let the women-folk pipe in too.