His work has become an essential reference for philatelists interested in stamps of the nineteenth century.

Until very lately I believed, as he did, that mental phenomena have essential reference to objects, except possibly in the case of pleasure and pain.

His regional maps and two topographical descriptions of Lower Canada are still considered an essential reference for knowledge of the territory.

Common Names of Insects - This resource is an essential reference for anyone who works with insects.

Confidence grows with experience, but a book like this, chockful with essential reference, will provide a solid stepping-stone.

They are still essential references in the field for the groups that are treated in their pages.

This multi-volume series is an essential reference for judges and lawyers.

But this work is an essential reference, assembling in one place information formerly available only in medical and historical textbooks.

This is an essential reference for anyone designing user interfaces, and it's well written to boot.

The handbook, which continues to be made available online at no cost by the successors to RCA, is considered to be an essential reference.