OK! You want the Manual of the Planes to start with. It's got an entire section on the Feywild, which parenthetically was written by John Rogers, mastermind behind the TV show Leverage, which has an RPG based on it. (But don't buy the Leverage RPG, it won't help you with fey pact warlocks.)
The aforementioned "Performing the Pact" continues to be useful for the same reasons it was useful for star pact warlocks -- good roleplay advice, and some nice mechanical benefits.
Unfortunately, there's no fey pact equivalent of the really good star pact warlock article. There is, however, a great Feywild article: "Court of Stars: Prince of Frost." It's mostly background material about the Prince of Frost, a powerful evil fey being. There are also several pages in the back directly addressing warlock pacts with the Prince, including both roleplaying material and mechanical crunch.
Similarly, Dungeon had a Creature Incarnations article on Fomorians. It's not oriented towards warlocks, but if you're looking for great stuff on the Feywild, it's worth reading.
If you're looking for adventures, King of the Trollhaunt Warrens takes place in and around the Feywild. I don't find it hugely evocative of the fey spirit except maybe in the last section, but it does offer one perspective. Similarly, the RPGA module The Lady in Flames is strongly Feywild flavored.
Continuing to third party resources, Goodman Games comes to our aid again, with Creature Cache 4: Fey Folk. It's purely a monster book, so if you want more Feywild denizens, it's where to go. Kobold Quarterly has a fey-themed adventure, Courts of the Shadow Fey, which might also be useful.
Partial cover and concealment are identical to cover and concealment.
It's a confusing terminology change that happened part-way through 4e's lifespan, not the addition of an extra level of cover and concealment. The modifier "partial" was added to 'normal' concealment --probably in a well-meaning effort to clarify things-- and both the Rules Compendium and the Online Compendium parenthetically note that it is "sometimes simply called 'concealment.'" (RC 222)
The Online Compendium's glossary entry on cover still hasn't appended the modifier "partial," nor does it note that this word might sometimes be added, but the Rules Compendium uses the phrase "partial cover" and adds that it is "sometimes simply called 'cover.'" (RC 220)
Keep an eye on "superior" and "total," too.
There is equal potential for confusion in the upper echelons of cover and concealment: better cover is called "superior cover," while excellent concealment is "total concealment." I see no particular reason these two similar effects should have different modifying terms, and am surprised that only one item (the Nightmare's Keen Senses from DR 393) is currently confused by this (according to a search of the Online Compendium at the time of this post).
Best Answer
WotC hasn't produced a Far Realms book yet, much to my sadness, and none of their books provide a ton of star pact material. The planar books are more useful for infernal or dark pact warlocks, unsurprisingly.
There are a few good Dragon articles. "Performing the Pact" is a general guide to roleplaying pacts and patrons; it contains hints and one sample patron for each pact, including the star pact. There are also feats and background options appropriate for a star pact warlock. "Wish Upon A Star" is nearly essential for a star pact warlock. It ties the stars into your favorite Cthulhoid entities and provides a ton of roleplaying advice useful for both GMs and players. It's also got a lot of mechanics: feats, lots of powers, a paragon path, and an epic destiny.
The Eberron Campaign Guide is worth looking at for the material on the Cult of the Dragon Below. They're the aberrant-worshippers of the setting. I was particularly amused by the Finches, a family of backwoods darkness worshippers, with names like Thelonius and Lucretia. Lovecraftian if anything ever was. The monsters of Khyber and Xoriat fit Lovecraftian themes nicely as well.
As far as third party material goes, Goodman Games has a book of Cthulhu monsters, Critter Cache 6: Lovecraftian Bestiary. Issue #2 of their magazine, Level Up, has monster stats (which might be duplicated from the Critter Cache book) and a Cthulhu-oriented adventure.
Besides the 4e material, I'd at least think about some of the Lamentations of the Flame Princess books. The main rulebook/boxed set probably isn't that applicable, but I have Death Frost Doom and some of his Pembrooktonshire material: it's all very creepy and not tightly tied to any one system. Tower of the Stargazer has a very promising title.