Eligibility to appear for the UPSC exams if graduation has been completed directly

You are question is very general in nature. There are many posts that are published by UPSC. The eligibility criteria will be different from one job to another jobs.
1. If the eligibility criteria are given as graduate from any recognised university or equivalent, your qualifications will be sufficient and you can apply for the same. If it says a graduate following the 10 + 2 +3 pattern, you are not eligible and your application will not be considered. So you have to see the qualification required for that post, and then you can apply. Direct graduation holder is also eligible for some posts.
2. The books you have to study will also depend upon the subjects you choose and the type of examination you are going to write. If it is for civil services, you can get the syllabus from the advertisement and many books are there covering those subjects.
3. You should understand the subject well and you should have good basic knowledge of the subject. Take the books you studied in your graduation and read them to understand the subject but not from an examination point of view. Solve model question papers as much as possible. Without understanding the subject don't jump topics.

The eligibility qualification mentioned in UPSC prospectus for exams is a graduation degree. If a person even does class 12 from NIOS and then does graduation then he is also eligible for UPSC. In the same way if a person has done a graduation course without class 12 then one is eligible for appearing in UPSC.
So the eligibility is graduation and if that degree is an approved one then there is no problem in applying for UPSC exam.
There are a large number of guide books that students use for preparing for this exam.
These books cover different subjects and basic knowledge in various areas like logical reasoning, general knowledge, science, history, arts and culture, geography, political science, commerce, economics, law, environment, and many more.
For preparing for UPSC exams a long term strategy is required because the syllabus is very wide and one has to work hard and cover a lot of subjects.
It is said that students who want to qualify in UPSC exams have to work on it right from their high school level and think and act in that direction.
Current affairs, business scenario in the international market, political and diplomatic relations, foreign policy, defence and army, agriculture and many other areas have to be well understood by the candidates.
Those who want to appear in UPSC exams have to focus in the studies from a larger perspective and it is necessary to solve the question papers of earlier years to find out one's speed of answering and things like that.
Experts believe that the studies for UPSC exams cannot be completed in one round because until the students repeats the material a few times it would not stay in his mind and he will not be able to reproduce the knowledge for a correct answer. So it is usual for the candidates to study at least 8-10 hours per day for a significant period.
Some common books advised are -
1. For general studies 'Prelims Manual for GS by Manohar Pandey' is a good choice.
2. Cracking the CSAT by Arihant Experts is a good resource.
3. Fundamentals of Essay and Answer Writing – by Anudeep Durishetty is a good book for developing writing power.
4. World History – by Krishna Reddy.
5. International Relations – by Pavneet Singh.
6. Environmental Studies: From Crisis to Cure by Rajagopalan.
7. Science and Technology – by Ravi P. Agrahari.
8. Indian Economy – by Nitin Singhania.

The eligibility criteria for appearing UPSC examination is the graduation regardless of the streams undertaken by the aspirants. There is no specific mention of any clause debarring the aspirants if they have done straight their graduations ignoring the intermediate rout. However, the aspirants should ensure that the degrees acquired must be through the recognised universities accredited by UGC so that they would not face any issues at the later stage.
However, the aspirants need prudent planning and sustained efforts in clearing both Preliminary , CSAT, Optional Papers apart from final interviews where the board members would assess the inherent potentials of the aspirants through their brain storming questions. To sum up, it needs guidance of some professional institutes such as Byju's, Dristi, Rao's Study Circles etc taking care of your preparations at the different stages, giving you proper feed backs time to time for the better preparation of the subjects so that your timing is saved through their regular guidance. Moreover they conduct Mock Series Tests for enhancing the confidence levels of the aspirants. The following points written points could be helpful to the aspirants-
1) Go through the entire syllabus- The aspirants should go through the entire syllabus so that they would be familiar with the patterns of questions being set from the different chapters providing them better ideas of handling of their preparations.
2) Proper Revision- The syllabus of preliminaries is quite exhaustive and scattered and hence regular revision is required to avoid the last minute stress in their upcoming examinations.
3) Proper notes are to be made - Notes should be made on the proper format so that it could be easier for the aspirants to memorise the materials during their preparation sessions. However, it should not bulky either.
4) Avoid studying new portions prior to fortnight of the examination- Ensure that no new portion is to be read just before the commencement at least a fortnight prior to two weeks so that no undue stress develops with such activities. This could deprive the aspirants to pay proper concentration to the syllabus materials which need last minute revisions.
5) CSAT should be given equal weightage- It must be taken at par with the general studies. To ensure that preparations are going smoothly, revisions of last year questions accompanying their solutions set for at least five years should be taken up without fail.
6) Current Affairs are the most time consuming paper- The aspirants must devise some boundaries with the proper time allocations for this subject - Giving too much time or less timings for this subject could be the cause of resentment for the aspirants and hence a balanced approach needs to be maintained.
7) Choose the optional papers wisely- Irrespective of the background of science graduates, engineers or medical graduates, selection of papers are to be made which could reward the aspirants in respect of mark scores. Philosophy, Geography, History etc could prove to advantageous rather than taking Physics, Mathematics, Chemistry etc.