The TI-59 was a very interesting calculator but like all things electronic, it aged quickly. Its planned successor was the TI-88, project which reached the final pre-production stage before being cancelled. More details about this can be found at the TI-88 page on the DATAMATH Museum site. Manuals can be downloaded from there also.
Very few prototypes are known to exist, less than 20 in total. I am the lucky owner of one of those. “Lucky” in a manner of speaking, whenever I turn it on I am afraid I will beak it!
Here are some images