Woe be to him that reads but one book - meaning?

Woe be to him that reads but one book

The construction is obsolete: it has however become frozen in use.

Woe was originally an an interjection

OED:

Woe: A. interjection (a cry). 1. Used to express grief, pity, regret, disappointment, or concern.

It was not often used after the 18th century, except in literary or poetic use.

1619 A. D. B. Court of James, I 52 Oh woe! oh shame; alas,..what tongue is able to express, how..grievous it is?

Woe could also be used to explain why you were sad, grieving, concerned, etc:

2. With following clause or phrase expressing the object of the lament. Now archaic and literary.

a1720 W. Sewel Hist. Quakers (1722) vi. 336 Wo that ever I was Father to such wicked Children.

But in your example, it was originally an adverb but became a noun:

B. adv. With a dative (or, later, with to, noun, or objective pronoun as complement), with or without a verb of being or happening, in sentences expressing the occurrence of distress, misfortune, or grief. Now archaic. Arising as an adverbial use of the interjection (see sense A.) with the dative, although in later use probably often interpreted as a noun Now only in fossilized use (in e.g. woe is me ) and archaic use.

1. In prophetic or denunciatory utterances of the type of Old English wā biþ þǣm mannum [edit = literally “Woe be to those men”] ‘affliction or grief shall be the lot of the men’; woe be to us ‘may distress or misfortune afflict us’; woe is him ‘cursed is he’.

1636 Earl of Manchester Al Mondo: Contemplatio Mortis (rev. ed.) 162 Woe is him whose bed is made in hell.

1781 J. Tucker Treat. Civil Govt. ii. ii.150 Woe be to the Country, which happens to be cursed with a successive Race of Heroes.

Thus

Woe – a curse/deep misfortune/sorrow

be – shall be

to him – (dative = put upon him)

that reads - who reads

but - only

one book

Or, in Modern English: “If anyone only reads one book [and no more], he will regret it deeply [or, he will be greatly disadvantaged.]”

"Woe", in this sense, means "misfortune". So the beginning of the sentence means "Misfortune will come to him".

"But", in this sense, means "only" -- "who reads only one book".

The quote implies that reading only one book will give the reader a very narrow view of the world, and will lead to ignorance, prejudices, and misunderstandings.

It's vaguely possible that Herbert was trying to say that people should read more than just the Bible.