Mounted Or Unmounted

My question is this, say I have the grand daddy of most stamp Collections inherited, and has been sitting in the attic in a box, it is an old 'book' filled with a lot of stamps from around the world, I understand that some may have value. Because they are so old,(1800's) do I leave them mounted, or soak them off the backing ? I plan to sell them.

Leave them mounted. Taking them off, even if they are just cut from envelopes may remove useful information in the postmarks and cancellations that will give details to its history.
If they are on envelopes, absolutely leave them on. Covers are worth more than singles (but may be a little harder to find a buyer, it's worth shopping around first then).
Just because they are "old" though, doesn't guarantee value. Some stamps were still printed in the hundreds of millions during the "early years" of stamps.

If you can post a few photos of the pages, we could comment more effectively.

Can you post a scan of a page or two? That would help us advise you. Many older stamps can be irrevocably damaged by soaking or other means of removal. Also, you'd likely have an easier time selling the whole book as is, rather than by breaking it up. To break it up into smaller lots you'd need expert advice.

Beware of the just because they're old fallacy they have $$ value.

I wish you the best, but you should also temper your expectations a bit, since it sounds like they've been in an attic for some time.

Environmental conditions usually encountered in attics - extreme heat/cold/humidity are not good for preserving stamps over time. Especially unused stamps with gum.

i will echo what the others before me have said. leave intact. post scans of some of the pages of items. age does not translate into value. condition plays a large part.

My opinion...yes it could be that it is best to remove them. But until you post some images and let some of the experienced folks here better understand the scope of what you have and offer opinions; leave them alone. But the first step is to post some images and get feedback.
Don