one person has requested from me a large flat childs book and a much smaller thicker book. is there any way to send them together or do i need to post them seperately? thanks for your help!

If you have some extra plastic bags, bubble wrap, newspaper, etc to stuff around the smaller book you could put it on top of the child's book (which I would reinforce with some cardboard or other stiffener).  Then pack the "filler" around the smaller book and wrap the whole package that way.  This would work really well in a box, but can also be done if using paper.

This is of course if you don't have a bubble mailer you can recycle that is large enough to use for this package.

Can you put them in a box? Sometimes if I don't have a box I will wrap them separately and make sure they are nice and secure in their own wrapping, then tape them securely together so that I only have to pay postage on 1 package, but they are a little more protected being in their individual wrapping. I also have used Heather's method of adding padding along the sides of the smaller book to make its shape match the bigger book.

I would say you probably should use a box, particularly if the large book is thin, because in an envelope it will be prone to getting bent at the corners more than if it were just the large book alone.  But you really do want to try to combine them, because it will probably be much cheaper that way.

What are the actual dimensions?

I would cut 2 pieces of cardboard slightly larger than the size of the larger book.  I'd wrap it one piece of cardboard, the larger book, then the smaller book.  Pack some packing peanuts or bubblewrap solidly around that smaller book to fill in the space.  Second piece of cardboard on top.  Wrap it all up in a plastic bag and tape securely.  Then, I'd wrap it in some brown kraft paper and tape securely again.

I've also just thrown in an extra book or two if I need to fill space in a package like that.  I've always got lots of extra books around!