Serpents Among the Ruins by DRGIII I've read about 4 times...I read it every few years. I've posted before what a kick @$$ book that is....so I won't digress further on that.
Also have read Vanguard twice, the whole thing. I remember reading about it being a new series...I thought, "OK, they got a lawyer, a writer, and old fart for the cranky doctor and some Orion as a space trader as main characters....this is gonna suck."
I was wrong. Probably the very first book and Reap th Whirlwind were my two favs in the series. Vanguard I think captured the Essence of Trek.