LOL. The PCs break into the mansion and, aside from a little trouble with the dogs, they encounter no difficulty in stealing the journal. The old woman and her daughter aren't home that night, apparently. (But why? ...) The PCs will not be able to read the journal because it's written in a cipher, and they don't have the secret codeword. They travel back to the city and give the journal to their boss, who gets angry and orders them to do whatever it takes to get the codeword. So, the PCs go back to the mansion and kidnap the lady alchemist's daughter, leaving a ransom note somewhere in the house. She agrees to meet with them, gives them the codeword, and takes her daughter back. If the PCs are resourceful enough to read the journal, they discover that it contains, not a formula for a healing potion, but an elaborate account of how the woman, who, it is revealed in the journal, was once the guild boss' lover, cheated on him for years with another man, the head of a rival crime family, and eventually betrayed the guild boss, telling the other man all of his secrets, which cost the first guild hundreds of thousands of gold pieces over the years. At the very end of the journal, there is a personal message to the PCs boss: "So you see ABC, I didn't leave you because I wanted to be alone and focus on my work... I left you for XYZ (the rival mob boss). There is no potion formula, there is no potion, there is only my revenge." Or something along those lines. Before this, the journal (which is really an extended letter to the PCs boss, given over intentionally by the old lady) could also go into detail about how the boss is evil, treated the woman cruelly, etc, etc... thus putting the PCs in a sort of moral dilemma. Do they go back to the old woman and ask questions? Do they side with her? If so, too bad, because she's really a necromancer and the "daughter" is her apprentice. Do they give their boss the journal? That might put them on the track of the real secret... that their boss was never involved with the lady, it was his twin brother the chaotic evil illusionist.
Forgive me, I just saw The Prestige.
I really like some of the other suggestions, here, especially jollyninja's.