Hi.  This is a pretty vague question but:  when I was a kid, I read a
sci-fi novel whose title and details escape me.  The basic premise, if
i remember right, was that the planets of the solar system had been
replaced by dyson spheres.  so one at the radius of mercury, one at
the radius of venus, etc.  no one knows much about how this came
about, and what is at the center etc. etc., so an assassin named the
lion (and described as being magnificently ugly) and some other people
get hired by some billionaire or ai or something and try to go to the
center.  note that i could be wrong about any of these details.  any
idea as to the title of the book?

Dear ajithr,

The book you have in mind is Colin Kapp's "Cageworld" (also known as
"Search for the Sun!"), first part of his series of four Cageworld
novels.

These excerpts of summaries show clearly that "Cageworld" is the book you remember:

"The five protagonists are hired by a mysterious man/machine to search
for the computer system that runs a completely redesigned solar system
involving concentric Dyson spheres. Their quest takes them inwards
from the 'mars sphere' towards the sun (which no-one has seen in
generations since they live on the *outsides* of the spheres)." [1]

"The stories themselves dealt with a group of explorers who flew from
one dyson shell to others, something that had never been done. In the
first book they wanted to get to the center of the solar system to see
the mythical sun. I don't remember what the other missions were. There
were at least three characters - a master assassin, an aquatic mutant
who could generate bio-electric shocks and a master showman who was an
expert with holography." [2]

In case you'd like to purchase this novel: Although "Cageworld" is
currently out of print, there are various offers for used copies at
Amazon, many of them inexpensive:
http://www.amazon.com/gp/offer-listing/0879978589/002-5650762-1910404

I hope this answers your question!
Regards,
Scriptor


Sources:

[1] Amazon.com: Cageworld 1: Search for the Sun
http://www.amazon.com/gp/product/0879978589/002-5650762-1910404

[2] "Re: Colin Kapp's Cageworld series", by Peter Meilinger.
<rec.arts.sf.written> (9 June 1997) via Google Groups
http://groups.google.de/group/rec.arts.sf.written/msg/e58faa63332ddd70?hl=en&

Wikipedia: Dyson Spheres in Fiction
http://en.wikipedia.org/wiki/Dyson_spheres_in_fiction


Search terms used:
cageworld summary
://www.google.de/search?q=cageworld+summary&hl=de&lr=&newwindow=1&safe=off&start=0&sa=N
cageworld "colin kapp"
http://groups.google.de/groups?q=cageworld+%22colin+kapp%22&start=20&scoring=d&hl=de&lr=&safe=off&
"concentric dyson spheres"
://www.google.de/search?hl=de&newwindow=1&safe=off&q=%22concentric+dyson+spheres%22&btnG=Suche&meta=