das Ries
ream
A unit of measurement for paper, usually consisting of 500 sheets.
Für das Projekt benötigen wir ein ganzes Ries Druckerpapier.
For the project, we need a whole ream of printer paper.

ream / Ries
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ream
A unit of measurement for paper, usually consisting of 500 sheets.
Für das Projekt benötigen wir ein ganzes Ries Druckerpapier.
For the project, we need a whole ream of printer paper.

The two meanings are very different. The context makes clear what is meant. If the topic is paper, sheets, or printing, the unit of measurement is meant. If the topic is geography, a crater, or southern Germany, the landscape is meant.
Ich habe noch ein halbes Ries Papier im Schrank.
I still have half a ream of paper in the cupboard.
As a unit of measurement, 'Ries' (ream) is used almost exclusively in connection with 'Papier' (paper). Typical phrases are 'ein Ries Papier kaufen' (to buy a ream of paper) or 'ein Ries Druckerpapier bestellen' (to order a ream of printer paper).
Der Büromateriallieferant hat uns zehn Ries Papier geliefert.
The office-supply provider delivered ten reams of paper to us.
In the geographical meaning, 'Ries' is a proper noun. It is almost always used with the definite article 'das' and is often specified with 'Nördlinger': 'das Nördlinger Ries' (the Nördlinger Ries).
Das Nördlinger Ries entstand vor etwa 15 Millionen Jahren.
The Nördlinger Ries was formed about 15 million years ago.
book
'das Buch' and 'das Ries' both have to do with paper, but they mean very different things. A 'Buch' is a work with pages that you can read, as a printed or digital object. A 'Ries' is a unit of measurement for paper and usually means 500 sheets. Learners may confuse the words if they are only thinking about paper. For something to read, you say 'Buch'. When talking about a quantity of paper for purchasing or printing, 'Ries' can fit.

Für die Kopien kauft ihr im Schreibwarenladen ein Ries Papier.
For the copies, you buy a ream of paper at the stationery store.
Zu Hause liest die Ärztin heute ein Buch über Pflanzen.
At home today, the doctor is reading a book about plants.