Translation:It is a book about hot nights and cold mornings.
Ablative has merged with genitive some day in Balto-Slavic or so. The ablative sense "from" is present in the genitive case.
That is different from the best known ablative - the one in Latin. That one has merged with instrumental. So The Czech instrumental case contains some of the ablative stuff you might be familiar with from Latin.
I do not know any well-known Indo-European language (existing or at least well preserved) that would preserve all 8 original cases.
Sanskrit is well preserved and has all 8 original PIE (Proto-Indo-European) cases.
Lithuanian officially has 8 cases, but not the ablative (which merged with the genitive, as you said). The 8 cases now include the illative, which emerged as a new case in the evolution of Lithuanian (most likely by merging the preposition "na" as an ending), but it's a shaky case, often replaced by the locative in standard language, but used a lot in informal speech and in dialects.
At some point, Lithuanian had 10 cases, where the allative and adessive existed next to the illative, all created from formerly separate prepositions, not inherited from PIE. Otherwise, such cases are common in Uralic languages.
I presume he means "It is THE book about..." instead of "It is A book about..." And since Czech has no articles, unless "je to" is only used in a sentence which, when translated to English, would use the indefinite article (which I doubt), both translations should be correct.
What you're probably getting at is "Je TA kniha o..." (this word order would be a question), or "TA kniha je o...", which would be a demonstrative, and as such would most likely be translated to English as "that", but I guess "the" could work too.
My question is about "je to..." in this usage. Why is this not interpreted as a question? Is it permitted to use "To je...." and if so what if any difference does it make to the meaning? English makes a clear demarcation between "Is it..." (question) and "It is..." (statement) whilst the other wording in the sentence stays intact. It is not a "rule" just a convention so one knows where one is.
Both are commonly used for statements.
To je... would be more often used when pointing to something. The thing you see here is ...
Je to... is typical when you continue the discussion about this entity. You add another fact, that it is ...
Also used for answers: Je to velké? Ano. Je to velké.