The link nks put up has a very nice discussion. The simple method is to start in book2 and type "=", then just click into book1 and click the cell you want to link to. Finally, just press enter. Ideally, you already gave the source cell a name, so the formula will use the name.

Please don't do anything more in that formula. Life is complex when you mess with this, so do your manipulation in another cell. Since any work you do in book2 can be broken by changes made in book1, you need to know that the layout of book1 is stable, and that only qualified people are going to edit it.

Something else I have done is put all the link formulas in one sheet on book2. That way, I only have to look in one spot to identify issues with the links. I then use formulas within book2 to get the information where I need it.

I find things work best when both files are in the same folder. That way, you can move or copy both without having to edit the formulas. Just open both book1 and then book2 in the new location and save book2. It will have the new file path in the formulas from then on. If the files are in different folders, that method won't work.

My advice is to avoid this where possible, but it can be safely done. Keep the links as simple as possible.