Get the OEM if you can. I have the Clymer as well as the real Merc one for my Merc 150 XR6 and there is no comparision in the amont of information between the two specfic to the motor i own. In the Clymer type manuals they try to cram way too many models into one book. The clymer i have for my 150 covers the following models( from the clymer website) into one 480 page book.
65 Jet, 75 hp, 80 Jet, 90 hp, 100 hp, 105 Jet, 115 hp (4 Cyl.), 115 hp Optimax (V-6), 125 hp, 135 hp, 135 hp Optimax, 140 Jet, 150 hp (Carburetor Equipped), 150 hp (EFI), 150 XR6, 150 Mag III, 150 hp Optimax, 175 hp (Carburetor Equipped), 175 hp (EFI), 175 hp Optimax, 200 hp (Carburetor Equipped), 200 hp (EFI), 200 hp Optimax, 225 hp (Carburetor Equipped), 225 hp (EFI), 225 hp Optimax and 250 hp (EFI).
My Merc manual covers a more focused model range in more pages. The merc book is well over 500 pages and it covers the 150 xr6, xri and efi and 200 carb and efi ( no opti coverage) from 99 to 01. All of the motors covered are 2.5l blocks and are very similar. The info given is way more detailed and it includes part numbers and troubleshooting info not given in the clymer manual. It was also 5 bucks cheaper then the clymer at my local dealer.
Here is a link to a place to order a factory one from
http://www.marineengine.com/manuals/johnson/order_1980-present.html