"This book is big."
Translation:यह किताब बड़ी है ।
Are you sure? I do not know any Sanskrit. But I cannot find यह or वह in Sanskrit dictionaries. Looking up «this»/«that» the other way around, English-Sanskrit, does not yield any words that look similar enough.
My guess would rather be that those are direct Devanagari copies of the spelling in Persian script: يہ and وہ respectively. At least it seems plausible with the long history of Hindi/Urdu/Hindustani (or whatever name one might prefer) being written in (an adapted) Persian script, before Devanagari was used for it as well.
It would even explain why the pronunciation does not fit. In the scripts from the Perso-Arabic family vowels are often not very clearly indicated. Here it's just a «small h» at the end indicating a long vowel. It just doesn't have to be a long «a».
«ये किताब» would mean «these book», so it mixes plural with singular. For singular use «यह» (yah, «this»). It is indeed pronounced the same way as «ये» (yē, «these»), so it's easy to confuse at first. But you will quickly get used to it.
A similar historically grown spelling is found with «वह» (vah, «that», pronounced more like vō), which again has a plural form वे (vē, «those»).
The good news is the spelling does not get worse in Hindi (AFAIK). This is already a big exception, most of the time Hindi spelling is quite strongly connected with pronunciation. The biggest other thing is to know when a short a is written but omitted in pronunciation. This can be annoying, but it's still better than most European languages and certainly miles away from the big mess of English spelling.