Upvote:2
The Western Europe edition will hold lots of information you are not going to use, which is a waste of money and carrying weight but you might use it at other times. But the best reason not to buy this book but the France or even Paris book as well as an Italian is that the books covering a whole continent do not have much information on any location while the more limited area books have much more information on the same locations.
These days I would go without buying a guidebook for a few days of travel. It is very likely that you can find a (Lonely Planet) guidebook covering the area in a library or as cheap secondhand in a shop near to you, to select the locations you want to use. But while traveling you can better use internet to get you the hostels.
And visit a good rail site to get your travel information. Seat 61 might be the best site: http://www.seat61.com/Paris-to-Milan-by-TGV-train.htm or try the French or Italian national sites.
Upvote:2
If this is your first trip to Europe (it sounds like it is?), and you're not really sure where you should go and what you should see, then I think the LP Western Europe is a very reasonable choice to start with. (An alternative would be the somewhat misnamed Mediterranean Europe, which also covers Paris and Milan, but focuses more on the southern half of the continent.)
Both are designed pretty much exactly for people like you, and while it's not going to take you far off the beaten track, it's going to give you a pretty good idea of what's possible, how much it will cost, and a bunch of sample itineraries to work off, all in a format that's easier to deal with than a million random web pages. You can cut costs a bit by buying the PDFs instead, or cut weight by buying the guide and physically cutting out the chapters you end up deciding you'll never need.
(Disclaimer: I used to work for Lonely Planet.)