star trek paperback and vttbots in it

Something like 20+ odds years ago I bought a star trek tos book and it was about the seaview. I can't remember the title. Mr Penn who had the watch that could go through time went into the 23rd century and brought back the enterprise to vttbots time. I also can't remember the story line but really liked reading it. Penn did not reveal to nelson and crew who kirk and his crew were from.
Do remember one part of the book were nelson was trying to conceal kirk on the electronic room. Can't remember why on that. So whenever Penn was done with enterprise crew he sent them back in there own time. Don't remember even towards the end of the book if nelson and crew knew kirk was from the 23rd century.