“Best known as an ace guitarist (for Molly Maher, Randy Weeks) and a producer/engineer (Trampled by Turtles), Erik Koskinen should also be recognized as a masterful songwriter and rousing country singer with the release of his solo album, “Keep It to Yourself,” which he’s promoting Friday at Lee’s Liquor Lounge with Maher and Dave Simonett’s Dead Man Winter. The Upper Peninsula native comes off like a twangier Greg Brown or a moodier John Hiatt in down-and-out gems such as “Treat Me So Bad,” several rambling rockers and one downright horny gem, “Pretty Girls.”