This depends on what you want from your conversion.
It would be hard to find a book these days on this subject, and I can only suggest asking your local library if they know of one.
Most fishing boats already have living quarters such as state room, galley, etc. I don't know what your boat consists of, but if you are looking at a full conversion then it will be a total strip out and rebuild which will cost a lot of money, and is really unnecessary.
On the other hand if yours has the standard fishing layout then it may be possible just to convert what you have already, taking out a few bunks etc and getting rid of the fishing clobber such as the winch. Maybe a hand winch on the foredeck would be all you need to raise your anchor.
The fish store room I would keep as is, but remove the ice maker if there is one and convert this into a workshop with bench, vice, and tools plus spares cupboards for your engine oil and filters etc.
Also I would definitely fit a spare diesel tank to hold as much spare fuel as possible, fitted with a pump to transfer the fuel to the main tank. This would obviously be as low in the hull as can be fitted.
If your boat has only one main engine then that could be a problem for world cruising because if you had engine trouble far out that is not repairable you could be in serious trouble. A short mast on the stern would be a definite, fitted with a sail. This would keep your head up to wind in a strong blow if your engine did cut out.
You could also fit a main mast with sale which could give you some headway after engine failure.
Really, you need a twin screw (two engines) If not then look at fitting a wing engine through to another prop that's got sufficient power to cope with a heavy sea.
Get to know how much your engines burn per cruising hour and allow for this leaving plenty in reserve. Its surprising how the fuel goes down when your head is to the sea and blowing hard.
The other thing I would suggest is to draw out a layout of your boat on paper, then photocopy this a few times. While you are at home you can put down your thoughts on these drawings to get some ideas of what you want. When all is well then follow your plan to do the work.
One final thing I would suggest is to keep things as simple as possible. You don't need lots of electrics on board to go wrong. Simple bucket shower for instance. A chart table with a low light for another.
A large flat storage space underneath for all your needed charts along with book storage for the nautical books on the counties you intend to visit. I don't advise sailing blind without these.
With all the fishing clobber removed you will then have a better idea on what to convert or whatever you prefer to do with the space you have available..
I could go on for ever as there is lots to think about with a job like this. Pick up a few sailing books on fitting out. Its all the same really just a different power system.
Remember to keep things simple, Good luck on your venture.