Aragorn's father

[The Squatter of Amon Rûdh]

Well, again this can be found in appendix A to The Lord of the Rings, but yes. Aragorn was Isildur's direct descendant in the male line, the succession having remained unbroken among the Northern Dúnedain. No childless rulers are mentioned, although there was some strife between Arthedain, Rhudaur and Cardolan following the division of the kingdom, largely over the possession of Amon Sûl.

Most of the history of Gondor and Arnor can be found in Appendix A to The Lord of the Rings, sections ii, iii and iv. For future reference, basic questions like this ought to be posed in Novices and Newcomers.