Thanks (a lot) for your (last) letter.
Your last letter was a real surprise.
I was glad to get your letter.
It was great to hear from you! / It was great to hear that… / I was happy to hear…
Вы можете также извиниться за то, что не писали раньше:
Sorry I haven’t written for so long but …/ Sorry I haven’t been in touch for so long.
I'm sorry I haven't answered earlier but I was really busy with my school.
и/или упомянуть какой-либо факт из полученного письма:
I’m glad you passed your History test!
Sounds like you had a great time in London!
Great news about your…!