Q : What are the essentials of a lasting relationship?

Love, trust and readiness to sacrifice.

Q : What kind of signs do girls give you when they're interested?

You will get calls all the time - day and night. SMS too in your mobile....

Q : Which signs are pointing out that relationship is going to develop in a long term love and friendship?

Your financial stability

Q : Do you recommend any book which gives dating tips?

Experience is the best teacher. Any way are you in LOVE ?.
Read the famous book - How to avoid matrimony - too before venturing.