One of my Sims I am playing as is starting a self-employed career as a writer, and is struggling to get by. Since he has enough Lifetime Reward points, I am considering getting him the Profession Simoleon Booster LTR in order to make some extra cash in my royalties/stipend. However, according to the reward description it says it helps "professions". Does this include self-employed careers such as writing, fishing, etc?

Sorry if this has been answered before--Although I have been a member of MTS for a while now I usually just lurk and thus am not too familiar with actual posting etiquette in relation to the site. Hopefully i'm not repeating things.

No it doesn't work for self-employed careers. You can buy the Entrepreneurial Mindset one.

The biggest difference to my Writer sim was the number of books read. When he'd only read 10 books, he was earning royalties of §100 per week. Wow! Once he'd read 30 books, he suddenly went up to royalties in the §2000 a week range! Big difference!

The type of book you choose to write also makes a difference. There's a table showing the details at Carl's Sims 3 Guide.