In the first three sentences you need emphatic particles for them to make proper sense. The emphatic particles are used for emphasis or contrast, here you're contrasting what different persons do.
Ní léann SEISEAN leabhair, ach léimSE iad.
An léann TUSA leabhair freisin? (without the emphatic particle it means "Do you read books too [as well as newspapers, magazines, etc.])
Ní léann siadSAN leabhair, ach léimidNE iad.
Sin é an cailín nach léann a Hathair leabhair.
Although "cailín" is indeed a masculine noun, the fact that it refers to a feminine person overrides that grammatical detail, so you would still refer to a "cailin" as "sí" and use the feminine possessive adjective. The grammatical gender is still important, though, when you use adjectives and articles and form the genitive (e.g. an cailín beag, leabhar an chailín, leabhar an chailín bhig).