Je is a good choice because the book is feminine noun. But I would put it it behind On Želim kupiti knjigu danas, a on je želi kupiti sutra. These so called enclitics (the short forms of the verbs and pronouns) tend to stay always after the first accented word in the sentence or in the clause. Ja te želim videti. But if you omit Ja, then you must say Želim te videti.

1. We can say it is correct but see down signed with * 2. Little different word order: Želim kupiti knjigu danas, a on je želi kupiti sutra. Even much naturaly is if you put "ali" instead of "a": Želim kupiti knjigu danas, ali on je želi kupiti sutra. * somebody says "ju" instead of "je" (grammar has some rules for this). Želim kupiti knjigu danas, a (or ali) on JU želi kupiti sutra. Anyway, we don't see much sentence with "ju"; it's normal to use "je". So, you shouldn't bother with this. :) I hope this didn't confuse you.

Thanks for your answer and examples, Nez. By the way, I got your Christmas card. Thanks so much! :)

(Sorry for the late answer!) I completely agree with Ivana. I'd just emphasize that you put a comma before the conjunction "a". Also, you can also use "dok" ("while") instead of "a": "Ja želim kupiti knjigu danas, dok je on želi kupiti sutra." It's not a very common thing to use in spoken language, but it's used in writing. As for what Tina was saying about "je" vs. "ju": "je" is the normally used form, but if the word before it ends in -je or the word after is is "je" (3rd person sg. of "biti"), then it becomes "ju". In spoken language "ju" is used much more often and not according to standard rules. :P Examples of how it should be used: Saša ju je vidio. (not "Saša je je vidio."; "Sasha saw her.") Nije ju strah. (not "Nije je strah."; "She's not afraid.")