Suggestions for an inspirational quote/poem/verse to write in the Guest book of an Obituary?

I actually think it is best to avoid those sorts of things. The family will appreciate anything you say, and they will know that everyone is struggling to awkwardly express their condolences. But a sentence or two that is clearly not a pat phrase will mean more to them than will a copy/pasted quote from Bartletts or the funeral home quote books.

Just say something thoughtful, like, "We are deeply saddened to hear about Tom's passing. He was a dear colleague and friend to many here, and our hearts are with you as you mourn" or something like that.