The thought - say one found (murder hobbo'd the previous owner) a spell book as a multi-class wizard 2 / cleric 3.  To keep it simple, the spell book has leomunds-tiny-hut within it among other things.  as i read the ritual casting rule you should be able to cast this.  (see quote from PHB).  Now you can not cast a spell you do not have a 'leveled' spell slot for, but as a 5th level full caster multiclass you have 3rd level slots, so can one ritual cast this spell?    

PBH - Ritual Casting - You can cast a wizard spell as a ritual if that spell has the ritual tag and you have the spell in your spellbook. You don’t need to have the spell prepared.

Sage Advice - For a wizard to cast a ritual spell contained in their spellbook, do they need to read from the book or use it as part of the ritual?

No. As written, the rule doesn’t require you to read from the book. The narrative intent, however, is that the wizard is reading from their book, but the only mechanical requirement is that the wizard has the spell in their spellbook.

For wizards you don't need to have the spell prepared, you don't need any spell slots at all. The requirements for casting a ritual spell as a wizard are:

- you have the spell in your spellbook (not in a book you found, you will need to scribe it into your own book first, unless it's a magic book that let's you bypass that (such as the books from Tasha's).

And that's it. You don't need prep or spell slots. If it's in your spellbook you can ritual-cast it at the spell's base level. The spell functions the same, the only change is you add 10 mins to the casting time.

Also I might add: You can't even scribe a spell that you wouldn't have a wizard spell slot for into your spellbook. so as a lvl 2 Wizard you can't scribe LTH into your spellbook and therefor won't be able to cast the spell in any way - ritual or not.

what is keeping this caster or any wizard from making the new 'found' book their spellbook?    if your current book has (10) total spells and the new book has (14) total spells ... why spend the time money copying new spells into the old book, when you can spend less time and money copying old spells into the new book?  at some point in time the new book will have less spells or not as many new spells, so this make less sense, but early levels ... it makes sense.   

there are rules out there about if one lost your spellbook, even mostly mundane ways of replacing it with an enduring-spellbook is an option for character with time and money.    so why can one not just replace their current spellbook with the found one?   

An underlying assumption is that each wizard uses their own unique encryption and shorthand, so part of the copying process includes translating and learning the spell itself. You are more than welcome to use the blank pages in another wizard's book, but until you take the time to understand that wizard's personal style and quirks, those spells are basically just chicken scratch.

Having a spell "in your book" is code for "you understand the spell well enough to just need a quick refresher from time to time".

Quote from Falebe >>

what is keeping this caster or any wizard from making the new 'found' book their spellbook?    if your current book has (10) total spells and the new book has (14) total spells ... why spend the time money copying new spells into the old book, when you can spend less time and money copying old spells into the new book?  at some point in time the new book will have less spells or not as many new spells, so this make less sense, but early levels ... it makes sense.   

there are rules out there about if one lost your spellbook, even mostly mundane ways of replacing it with an enduring-spellbook is an option for character with time and money.    so why can one not just replace their current spellbook with the found one?   

Because "found" spells are in a notation system you cannot use for preparation or casting. The copying isn't just "copying" - it's scribing, translating it into your own notation system and testing so can understand it in a way that lets you cast and prepare it.

The only exception are the Tasha books because the magic of those lets the spells be usable on attunement.