"Does Will's letter to Tessa contain anything spoilery for Clockwork Prince? Should I read the letter before, or after, the book?"
I'm guessing you mean the Barnes and Noble special edition that has a letter from Will to Tessa in it.
All print first editions of Clockwork Prince in North America, the UK/Ireland and Australia/NZ are going to come with a letter from Will to his family, that he never sent, in the back. That's *ALL FIRST EDITIONS." So ANY PRINT FIRST EDITION, WHEREVER you buy it, will have the letter. Indie stores, Barnes and Noble, etcetera. (I have asked about kindle/etc versions, but it looks like they won't have it.) You can tell the first edition because it says "collector's edition" on the front.
* If you pre-order or buy within the first week or so, you will almost certainly get a first edition. It doesn't matter where you pre-order from. Beyond that I genuinely don't know how to tell you to ensure that you get one. Check the picture when you're ordering. If the cover says "SPECIAL COLLECTOR'S EDITION" that's a first edition.
As above, BARNES AND NOBLE in the US will be carrying their own special edition. It contains not just the letter from Will to his family, but ALSO an inscription from Will to Tessa in a copy of Tale of Two Cities. The splash sticker on the front says "love letter" and I guess you could think of it that way, but it's definitely an inscription in a book. I'd advise not reading it until you finish CP because I think it will mean more when the book's over. Otherwise I think it might just be confusing. (The edition is one of those 'it lasts till it sells out' things.)
Walmart is also going to be carrying their own special edition, so more 411 on that when I get more information. Their special edition is going to be Jem-centric and you can get it from them either in the store or online. [Because no one expects anyone to buy more than one copy of the book, ALL THIS SPECIAL CONTENT WILL BE POSTED ONLINE AFTER AN EXCLUSIVITY PERIOD OF BETWEEN 3-6 MONTHS.] My advice, just pick whatever your favorite it and get that, or get a regular first edition and read the content online when the exclusivity period's up.
*All these materials are being made available to international publishers and I will keep you updated as to how they will be available outside North America.