El y lo confusions
If the question is "donde esta el libro?" Then we can either say El es en la mesa or No lo sé. Why don't we say Lo es en la mesa ?
6 Answers
One thing had nothing to do with the other. (Él) está en la mesa: (It) is on the table
No (LO) sé: I don't know
This would work just as well as: Está en la mesa, and No sé.
"Él" refers to "the book", and it's implicit in the answer, so we don't have to say it. "Lo" refers to "I don't know IT ( the answer to your question) In Spanish that "lo" is also implicit, so it's not necessary in the sentence either.
If the question is "donde esta el libro?" Then we can either say El es en la mesa or No lo sé. Why don't we say Lo es en la mesa ?
¿Dónde está el libro? I would answer: Está en la mesa. You would not use ser. I would try to avoid using a subject pronoun to refer to an inanimate object.
The lo vs. él question has to do with subject vs. direct object.
Lo in your second statement does not refer to “el libro”, instead to knowing where it is. No lo sé. I don’t know it- Normally in English we would say I don’t know that, or more likely simply I don’t know, to express the concept. In English we love our subject pronouns, Spanish is much more fond of the object ones we can take or leave.
Now you could say- Lo puse en la mesa- I put it on the table. Here, it is an object, so you include it an use lo.
¿Dónde está el libro? I would answer: Está en la mesa. You would not use ser. I would try to avoid using a subject pronoun to refer to an inanimate object.
Way to go, bosque 
Welcome to the forum, pooja
The recommendation of the Academy of Language is NOT to use a personal pronoun for objects, things, when they are the subject of the sentence.
So to say:
el sofá es bonito. Él está encima de la alfombra.
is technically not totally wrong, but no native speaker would say it and is not supposed to.
We tend to substitute the personal pronoun here by an demonstrative pronoun, which is commonly used as a subject for objects. Or, preferably, no subject at all.
El sofá es bonito, está encima de la alfombra.
El sofá es bonito, este es verde y rojo.
I don't know if you can read this, but it states clearly that it is not recommendable to use either él or ella for inanimate objects.
Las formas tónicas él, ella, ellos y ellas corresponden a las terceras personas con sus respectivos números (singular y plural) y géneros (masculino y femenino).
Pueden desempeñar funciones de sujeto (él canta bonito), de término de preposición (la rosa es para ella) y de complemento directo e indirecto (los invité a ellos; le dieron una beca a él).
Cuando esas formas actúan como sujeto, se recomienda que se empleen solamente para referirse a personas: Pedro Pedregal es catedrático universitario; él también da clases en colegios.
Obsérvese que en la segunda parte del periodo, el pronombre él está haciendo las funciones de sujeto y alude a una persona: Pedro Pedregal.
For more info , read this
As a matter of fact, this is what the Academy says:
a) Cuando las formas de tercera persona él, ella, ellos, ellas funcionan como sujeto, solo pueden referirse a personas; por ello, cuando se hace referencia a cosas, en español no se emplea ningún pronombre personal explícito: He leído tus últimos informes Enhorabuena: son claros y ofrecen numerosos datos (no *ellos son claros y ofrecen...); así, en «La decisión de la entrega la señalan las Farc, lo mismo que el sitio en que ella se produzca» (Tiempo [Col.] 24.9.96), debió omitirse el pronombre ella o sustituirse por el demostrativo esta.
It has already been mentioned that the one has nothing to do with the other. So, beginning with “estar/ser”, “está is used here because it is a matter of location. A thing/person is “in” or “at” a laction, so “estar” is used. “¿Dónde está…?” is a big clue. The same thing goes for “¿A dónde va….?”. It’s a different matter with “from”. “¿De dónde es ella?”(“From where is she?”). This would require “ser”. “Ella es de…..”.
“Lo” is the masculine of “lo/la”. Since “book” is masculine, “lo” is used here. If the object were feminine (“mesa”), you would use “la”.
El libro está en la mesa. (whole answer)
Ël está en la mesa. (Él here could be a substitute or pronoun for el libro but it's not usual or even not grammatical)
Está en la mesa. ( Omitting the subject el libro or él is the most common thing)
El libro is a subject.
Lo is a pronoun that substitutes objects
Example.
Yo leo el libro
Yo lo leo.
