How often does Desna appear in books 3,4,5, and 6?
We have Father Z in Sandpoint for book 1
Then there is the Temple of Desna in book 2
I am asking, as I have a player whose character worships Desna, and I am looking to see how many things automically tie in, and where I might have to alter some locations
The deities themselves don't really get involved in this adventure path.
Desna's followers don't appear in Book 3. Parts of Books 4 and 5 require the PCs to return to Sandpoint, so the'll meet with Father Zantus again. In Book 6 the PCs may find an abandoned ancient temple to Desna in Xin-Shalast, but her followers are no longer there.
As Captain Morgan Mentioned, followers of Lamashtu appear in Books 3, 4 and 5, and it may giveyou the opportunity to get Desna involved as well.
While there aren't many actual connections to Desna it isn't a streach for a Desnan to see the Return/Rise of an ancient evil wizard whoms prime display of power is slavery is a direct affront to Desna's preaching of freedom and travel and that Varisia is one of the prime hubs of her worship. However I believe the original versions of the book had a gazzateer on Desna in book 1 with a new Prestige class as well as some unique summons on the summon monster list. You could easily have a unique follower or herald appear and press the issue or tell them they are doing good. Maybe bless their Starknife or grant the party a lesser wish.
I've tried to add in Desna for the sake of my Desnan warpriest a bit, as well as to up the stakes at a few junctures.
1) I designed a small dungeon called the Tomb of the Spherewalker, a half-giant Varisian hero from around the time of Magnimar's founding. The warpriest was able to meet his spirit within the tomb and receive his blessing, as well as receive access to the Spherewalker PrC without meeting all of the pre-requisites.
2) As Lamashtu's influence waned in the region, Desnan divine casters started to get a more direct connection to her, and were able to receive advice, although not necessarily when they think they need it. This really started happening when the party spent a lot of money remodeling Thistletop and the Catacombs of Wrath into places of worship for Desna.
3) During Book 3, the party begins to reach a level of influence that could easily draw the attention of Desna's servants, especially when they defeat Barl Breakbones and the Hag Coven (a disciple of Mokmurian and followers of Lamashtu, respectively). Mokmurian poses a VERY present threat to one of Desna's favored realms, and is specifically doing so from an angle that masquerades itself as a just one - reclamation of stolen land and works of slavery.
4) Personally, I think that Nightspear (one of Desna's servants) might be willing to advise a party on defeating Mokmurian. They may also be able to give background on The Scribbler, as he is and was a powerful servant of Lamashtu who once controlled a weapon of mass destruction only a few hundred feet from where he is resurrected.
5) If the party needs a bit of a hint regarding Runeforged weapons while they're invading Jorgenfist (if they suspect Karzoug's role in Mokmurian's planned invasion), Desna's servants might be in the best position to guide them within the library. Personally I think Pharasma is in a better position for that one, but most parties don't have Pharasman clergy members in them.
6) Once the Runelords are a more present threat, virtually any good deity would be looking for champions to handle the situation. They may not think it's a worthwhile situation to show their hand and directly intervene (that could go very poorly in a delicate situation), but showing up to indicate the scope of the threat the Runelords pose seems absolutely fine, maybe even granting a boon while they're at it.