Origins
A title is a label. The word comes from Latin titulus, meaning 'inscription, placard, sign' — a physical marker placed on something to declare what it is or who it belongs to.
In Roman practice, the titulus served many purposes. Scrolls bore a titulus identifying the work. Shops displayed a titulus advertising their trade. Condemned prisoners were paraded with a titulus listing their crimes. The most famous example is the inscription above the cross at the Crucifixion: the titulus crucis, reading INRI.
The word entered English twice — through Old English titul (directly from Latin, via the church) and through Old French title. Both streams merged in Middle English, and the word accumulated meanings like sediment.
Development
A book's title identifies the work. A person's title identifies their rank. A legal title identifies ownership. In each case the function is the same: a label that declares identity, authority, or possession.
The legal sense is the oldest in English law. Title to land means proven right of ownership — the documentary label that says 'this belongs to you'. Title deeds are the physical descendants of the Roman titulus: inscriptions that make ownership visible and enforceable.
Titular preserves the idea of a label without substance — a titular head holds the title but not the power behind it.