Hi Belli,
If one's German is acceptable, then the 'Die Pistole 08' book by Gortz is good. Purchased a copy a few days ago and after reading 'Lugers at Random', 'Luger', 'The Luger Story', 'The Dutch Luger', 'German Handguns' I still found info I didn't know about
But to answer the original question:
My first purchase would be 'The Luger Story', as it offers a good overall picture.
Next on the list is 'Lugers at Random', although it desperately needs a revision. But it shows a good overview of pistols and their perculiarities.
After that it's too the specialist books, depending on the topic you go for.