So there is a conflict between two rules here. As Matheus’s fine answer covers, tongues itself refers solely to speaking and listening, and any mention of reading and writing is conspicuously absent, compared with, say, comprehend languages which explicitly covers reading.
The conflict I mention, though, is with some much more basic rules:
Any character except a barbarian can read and write all the languages he or she speaks.
(Basics → Races → Races and Languages)
A literate character (anyone but a barbarian who has not spent skill points to become literate) can read and write any language she speaks.
(Skills → Speak Language)
So one could argue that when tongues gives you the ability to speak some language, it also (per this rule) gives you the ability to read and write it—nothing in tongues says it doesn’t do that, which could arguably mean this default rule is still in play. Contrast that with comprehend languages, which explicitly says you understand and can read languages you wouldn’t otherwise know, but cannot speak or write them. Comprehend languages has an explicit exception to the connection between being able to speak, read, and write a language, while tongues just leaves any mention of reading and writing out.
(Obviously, none of this applies if you are a barbarian who somehow cast tongues.)
The counter-argument, of course, is that the first is in the “races and languages” section, and is “obviously” talking about the automatic and bonus languages you learn as part of character creation. The second, equally, is in the section on the Speak Language skill, and so only applies to languages learned that way (though the description—the first bullet—explicitly discusses the state of affairs prior to spending any skill points on the skill). One could even argue that the repetition here is important—that the rule written in one of these places wouldn’t apply to the other so they had to write it in both.
The counter-counter-argument is that if they only meant for those rules to apply to automatic/bonus languages and Speak Language, respectively, why didn’t they just say that? They explicitly—twice—used very general language, and they don’t even make any mention of “knowing” the language—reading and writing explicitly hinges on “speaking” the language, and tongues definitely does that.
All in all, I suspect that tongues probably wasn’t meant to cover written language, but because of how the basic rule was written, it’s impossible to be sure. It is best, then, to ask your DM to make a ruling here. I have played games where tongues covers written language and I have played games where it does not, and I have played in many, many games where it never came up and never would have mattered anyway. (Actually, even in the games where we discussed this up front and rulings were made, I can’t recall it ever actually mattering in game there, either.) Language is rarely an critical part of adventures, and it is a very difficult thing for a DM to add to an adventure in a way that remains fun.
Puzzling out something written in an unknown language is a function of the Decipher Script skill, and that skill gets no bonus for knowing related languages, or for speaking but not reading the language in question, and the tongues spell doesn’t mention it. So if tongues doesn’t help you directly, it also won’t help indirectly.