Pre-compose, apply a mask.
For this newbie, how ?
At the moment:
Layer 1 original footage
Layer 2 Logo 1 (left)
Layer 3 Logo 2 (left)
Layer 4 - Silhouette
Do I need to separate the silhouette layer to another composition and animate it ?
Where does the mask get applied ? At the moment I've got a mask on the footage (book) layer but it's black and covers anything else, and if I invert it, then I can only see the book page.
Select your silhouette layer. Precompose it (moving all attributes into the precomp). In your main composition, mask it.
If what I've said doesn't make sense, you really need to start here first: Getting started with After Effects
Ok, try this (If I'm understanding what you mean):
Create a new solid layer above your silhouette layer, make it 50% transparent (temporarily) so you can see things under it.
Now use the Pen tool on this solid to create a mask where you want you silhouette to be.
Once you close the path only the solid color within your mask will be visible, so now make it 100% Opacity again.
Now on your timeline window, you might see Source Name, maybe a batch of icons, Mode, T, TrkMat, Parent... you're looking for TrkMat. If its not there then right click on Source Name > Columns > Modes. And your Mode switches will appear.
So on your silhouette layer (which should be below that solid), under TrkMat will be a 'None' box, change that to Alpha Matte, and boom, your silhouette will appear only in the area you created with the pen tool.
Stormy - that did it ! Thank you so much
You could've drawn your mask directly on the pre-composed layer like the others suggested, however sometimes you may use an effect that doesn't behave well with the mask (like transform, or maybe a distortion effect), so by separating your mask onto another layer, it works no matter what effect you apply, it just gives you more control, and also, if you wanted to now animate your silhouette for whatever reason, your mask will stay put where it's suppose to