The first book starring the kilt-wearing hero is called "Methuselah's Children". "Time Enough for Love" and "To Sail Beyond the Sunset" follow. The last book is great, but you will appreciate it even more if you are familiar with Heinlein's other books, as it mentions characters from nearly all of them.
Sometimes I think Robert Heinlein raised me into adulthood, for better or worse. There is one story in TEfL that is a real tear-jerker, even after repeated readings.