I’ve bought almost every Turkish textbook available in Dost and D&R that isn’t awful. There really aren’t a lot of good textbooks available in Turkey, and the ones that are available seem to be strong in one area, but weak in others.
If you want a simple, comprehensible, and rather complete understanding of Turkish grammar – a good base for learning the language – then I would recommend Orhan Doğan’s “Modern Turkish: A Complete Self-Study Course for Beginners”.
It lays out grammar and the structure of the language in a much clearer format than I’ve seen elsewhere.
It does not, however, have a lot of conversational language or dialogue practice. Thus, if you want to both understand grammar and learn spoken Turkish , a good companion would be Starting Turkish (Orhan Doğan’s again, Milet Publishing), which conveniently lacks almost any grammar explanations (but does a great job of introducing lively dialogues and presenting common spoken usages).