You need to restart the process of acquiring ritual spells via this mechanic.

The process of copying the spell into your ritual book takes 2 hours per level of the spell, and costs 50 gp per level.

The feat mentions this.

When you choose this feat, you acquire a ritual book holding two 1st-level spells of your choice

This occurs only when you choose the feat.

Afterwards the content of the book, a physical object, is governed by the process described above. So if you lose it, your only recourse is to find more spells and use the rule above to scribe rituals into a new book.

As a general comment, the gist of the game is for the referee to present a setting for the player to experience as their character. In this sense it is a pen & paper virtual reality. This relates to your question in that given the lack of specific rules, you should consider the issue as if you were really there. The Warlock getting his Book of Shadows (PHB 108) is an example of a specific exception to the assumption of a pen & paper virtual reality. In this case the character has a physical book filled with ritual spells. Lose the book and you lose the spells and have to spend time and money to make a new one.

Note that there appears to be no initial cost assigned to making a new ritual book. However, in the equipment list a spellbooks cost 50 gp, and a blank book 25 gp. Personally, I would charge a PC the spellbook cost for a new ritual book. However, if you are not comfortable with that then charge the player the cost of a book. Other than that, there is nothing to prohibit characters from spending additional time and money to make a backup book.

Also, this is consistent with how the Wizard spell book works on page 114, which is the closet relevant mechanic.

Of interest is that the ritual feat is the only way for a sorcerer to cast ritual spells, as he doesn't possess a ritual casting feature as a function of class as far as I can tell.

You Can Only Do What The Spell States

However, this does indeed mean you can use it deliver spells with the range of touch, telepathically communicate within 100ft, send it to a pocket dimension, etc, as they are all specified in the spell itself!

You Can Cast Find Familiar With the Ritual Caster Feat

The spell has the ritual tag, so therefore it can be cast with the Ritual Caster feat.

Other Options for Gaining/Casting Find Familiar

• Multiclass into Wizard
• Multiclass into Warlock
• Multiclass into Rogue with the Arcane Trickster archetype
• Multiclass into Fighter with the Eldritch Knight archetype
• Ring of Spell Storing (thank you to Mindwin)
• Ask your DM for a custom made magic item