No.

On your adventures, you can add other ritual spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up) and if you can spare the time to transcribe the spell.

You can inscribe spells that you find. Which implies they must be written down. As a warlock spells you know, you gain magically through your pact to your patron. Those you don't understand in a way that can be expressed in text.

This is also been answered by Jeremy Crawford here: http://www.sageadvice.eu/2016/04/26/book-of-shadows/

Work Around

If your DM allows scroll scribing/magic item crafting, you could create a scroll and then copy it into you Book of Secrets. This could be very expensive as Common items (Cantrips and level 1 spells) require 100gp, and minimum level of 3, all the way up to Legendary items (9th level spells) requiring 500,000gp and a minimum level of 17. 5th level spells (the max for a warlocks spell slots) are rare, require the character to be level 6 and spend 5,000gp.

Looks like any written source (via Twitter):

Matt Harrah: "...For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?"

Jeremy Crawford: "...Finding one written down."

A wizard's spell book, another warlocks Book of Shadows, or a scroll all qualify. I could see a DM making one carved on a wall of dungeon, etc, and it would count RAW and RAI.