Before a noun, 这 almost always takes a measure word in between. The pattern is 这 + classifier + noun, as in 这个 for a person and 这 + 本 for a book. When no specific classifier comes to mind, default to 这个.
register
In casual northern speech, especially Beijing, 这 is often pronounced zhèi before a classifier - a fused form of 这 + 一. So 这个 becomes zhèige, and 这 + 本 becomes zhèi běn. Writing stays the same; only the spoken reading shifts.
Wraps the lower-left as the walking radical (side-form of 辵). Originally 这 was a verb meaning to meet or come up to, hence the motion radical. Through grammaticalization the meaning narrowed to the demonstrative this - the thing approached, the thing right here. Same motion family: 进 (advance), 退 (retreat), 远 (far), 近 (near).
Inner 文 supplies a residual phonetic value for an older verb meaning to greet (yán/yàn in classical readings). The modern reading zhè comes from a Tang-era colloquial substitution and bears no live phonetic match to 文. Treat it here as historical residue; its writing meaning plays no semantic role.
